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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2018	May 09, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Trading Symbol	PIRS
Entity Registrant Name	PIERIS PHARMACEUTICALS, INC.
Entity Central Index Key	0001583648
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		53,974,184

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 109,521	$ 37,878
Short term investments	50,061	34,751
Accounts receivable	3,309	15,546
Prepaid expenses and other current assets	3,190	1,615
Total current assets	166,081	89,790
Property and equipment, net	4,445	4,034
Long term investments	2,583	9,922
Other non-current assets	130	130
Total assets	173,239	103,876
Current liabilities:
Accounts payable	1,934	2,452
Accrued expenses and other current liabilities	6,578	6,170
Deferred revenues, current portion	46,251	37,153
Total current liabilities	54,763	45,775
Deferred revenue, net of current portion	67,746	46,542
Other long-term liabilities	37	37
Total liabilities	122,546	92,354
Stockholders’ equity:
Preferred stock, $0.001 par value per share, 10,000,000 shares authorized and 2,907 and 4,963 issued and outstanding at March 31, 2018 and December 31, 2017	0	0
Common stock, $0.001 par value per share, 300,000,000 shares authorized and 53,974,184 and 45,017,062 issued and outstanding at March 31, 2018 and December 31, 2017	54	45
Additional paid-in capital	185,638	136,484
Accumulated other comprehensive loss	(5,973)	(4,695)
Accumulated deficit	(129,026)	(120,312)
Total stockholders’ equity	50,693	11,522
Total liabilities and stockholders’ equity	$ 173,239	$ 103,876

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
Preferred stock
Par value (in dollars per share)	$ 0.001	$ 0.001
Shares authorized (in shares)	10,000,000	10,000,000
Shares issued (in shares)	2,907	4,963
Shares outstanding (in shares)	2,907	4,963
Common stock
Par value (in dollars per share)	$ 0.001	$ 0.001
Shares authorized (in shares)	300,000,000	300,000,000
Shares issued (in shares)	53,974,184	45,017,062
Shares outstanding (in shares)	53,974,184	45,017,062

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Revenue	$ 4,152	$ 1,343
Operating expenses
Research and development	7,936	5,360
General and administrative	4,352	3,989
Total operating expenses	12,288	9,349
Loss from operations	(8,136)	(8,006)
Interest income	325	0
Other (expense) income, net	(903)	12
Loss before income taxes	(8,714)	(7,994)
Provision for income tax	0	0
Net loss	(8,714)	(7,994)
Foreign currency translation	(747)	51
Unrealized loss on available-for-sale securities	(531)	0
Comprehensive loss	$ (9,992)	$ (7,943)
Net loss per share
Basic and diluted (in dollars per share)	$ (0.17)	$ (0.19)
Weighted average number of common shares outstanding
Basic and diluted (in shares)	50,046	43,064

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating activities:
Net loss	$ (8,714)	$ (7,994)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation	134	100
Stock-based compensation	1,004	752
Other	8	0
Changes in operating assets and liabilities	38,907	32,912
Net cash provided by operating activities	31,339	25,771
Investing activities:
Purchases of property and equipment	(414)	(179)
Proceeds from maturity of investments	3,600	0
Purchases of investments	(11,479)	0
Net cash used in investing activities	(8,293)	(179)
Financing activities:
Proceeds from exercise of options	826	0
Proceeds from exercise of warrants	126	0
Issuance of common stock, net of issuance costs	47,207	0
Net cash provided by financing activities	48,159	0
Effect of exchange rate change on cash and cash equivalents	438	295
Net increase in cash and cash equivalents	71,643	25,886
Cash and cash equivalents at beginning of year	37,878	29,356
Cash and cash equivalents at end of year	109,521	55,242
Supplemental cash flow disclosures:
Net unrealized loss on investments	(697)	0
Property and equipment included in accounts payable	$ 65	$ 613

Corporate Information	3 Months Ended
Corporate Information	Mar. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Corporate Information	Corporate Information Pieris Pharmaceuticals, Inc., was founded in May 2013, and acquired 100% interest in Pieris Pharmaceuticals GmbH (formerly Pieris AG, a German company which was founded in 2001) in December 2014. Pieris Pharmaceuticals, Inc. and its wholly-owned subsidiaries (collectively “Pieris” or the “Company”) is a clinical-stage biopharmaceutical company that discovers and develops Anticalin-based drugs to target validated disease pathways in unique and transformative ways. Pieris' corporate headquarters is located in Boston, MA and its research facility is located in Freising-Weihenstephan, Germany. Pieris' clinical pipeline includes an immuno-oncology bispecific targeting HER2 and 4-1BB, an inhaled IL-4 receptor alpha antagonist Anticalin protein to treat uncontrolled asthma, and a half-life-optimized hepcidin antagonizing Anticalin protein to treat anemia. The Company’s core Anticalin technology and platform was developed in Germany, and the Company has partnership arrangements with a number of major multi-national pharmaceutical companies. As of March 31, 2018, the Company had cash, cash equivalents, and investments of $162.2 million. The Company expects that its existing cash, cash equivalents, and investments are sufficient to support operating expense and capital expenditure requirements for at least 12 months from the date of filing.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

The Company´s significant accounting policies are described in Note 2, "Summary of Significant Accounting Policies", within the Company´s Annual Report on Form 10-K for the fiscal year ended December 31, 2017. There have been no material changes to the significant accounting policies during the three months ended March 31, 2018.

Unaudited Interim Financial Information

The accompanying unaudited condensed consolidated financial statements included herein have been prepared by the Company in accordance with accounting principles generally accepted in the United States ("U.S. GAAP") for interim financial information and pursuant to the rules and regulations of the Securities and Exchange Commission (the "SEC"). Accordingly, certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to such rules and regulations. In the opinion of management, all adjustments, consisting of normal recurring adjustment, and disclosures considered necessary for a fair presentation of interim period results have been included. Interim results for the three months ended March 31, 2018 are not necessarily indicative of results that may be expected for the year ending December 31, 2018. For further information, refer to the financial statements and footnotes thereto included in the Company´s Annual Report on Form 10-K for the fiscal year ended December 31, 2017, which was filed with the SEC on March 15, 2018.

Basis of Presentation and Use of Estimates

The accompanying condensed consolidated financial statements of Pieris Pharmaceuticals, Inc. and its wholly-owned subsidiaries were prepared in accordance with U.S. GAAP. The condensed consolidated financial statements include the accounts of all subsidiaries. All intercompany balances and transactions have been eliminated.

The preparation of the financial statements in accordance with U.S. GAAP requires management to make estimates, judgments, and assumptions that affect the reported amounts of assets, liabilities, revenues, expenses and the related disclosures at the date of the financial statements and during the reporting period. Significant estimates are used for, but are not limited to, revenue recognition; deferred tax assets, deferred tax liabilities and valuation allowances; fair value of stock options and various accruals. Management evaluates its estimates on an ongoing basis. Actual results and outcomes could differ materially from management’s estimates, judgments, and assumptions.

Cash, Cash Equivalents and Investments

The Company determines the appropriate classification of its investments at the time of purchase. All liquid investments with original maturities of 90 days or less from the purchase date are considered to be cash equivalents. The Company’s current and non-current investments are comprised of money market, asset backed securities, government treasuries, and corporate bonds that are classified as available-for-sale in accordance with Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) Topic 320, Investments—Debt and Equity Securities. The Company classifies investments available to fund current operations as current assets on its balance sheets. Investments are classified as non-current assets on the balance sheets if (i) the Company has the intent and ability to hold the investments for a period of at least one year and (ii) the contractual maturity date of the investments is greater than one year.

Available-for-sale investments are recorded at fair value, with unrealized gains or losses included in accumulated other comprehensive loss on the Company’s balance sheets. Realized gains and losses are determined using the specific identification method and are included as a component of interest income. Approximately $0.2 million of realized losses were recognized for the three months ended March 31, 2018. No realized gains or losses were recorded for the three months ended March 31, 2017.

The Company reviews investments for other-than-temporary impairment whenever the fair value of an investment is less than the amortized cost and evidence indicates that an investment’s carrying amount is not recoverable within a reasonable period of time. To determine whether an impairment is other-than temporary, the Company considers its intent to sell, or whether it is more likely than not that the Company will be required to sell the investment before recovery of the investment’s amortized cost basis. Evidence considered in this assessment includes reasons for the impairment, the severity and the duration of the impairment, and changes in value subsequent to period end. As of March 31, 2018, there were no investments with a fair value that was significantly lower than the amortized cost basis or any investments that had been in an unrealized loss position for a significant period.

Concentration of Credit Risk and Off-Balance Sheet Risk

The Company has no financial instruments with off‑balance sheet risk such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that subject Pieris to concentrations of credit risk include cash and cash equivalents, investments, and accounts receivable. The Company’s cash, cash equivalents, and investments are held in accounts with financial institutions that management believes are creditworthy. The Company’s investment policy includes guidelines on the quality of the institutions and financial instruments, and defines allowable investments that the Company believes minimizes the exposure to concentration of credit risk. These amounts, at times, may exceed federally insured limits. The Company has not experienced any credit losses in such accounts and does not believe it is exposed to any significant credit risk on these funds. Accounts receivable primarily consist of amounts due under strategic partnership and other license agreements with major multi-national pharmaceutical companies for which the Company does not obtain collateral.

Fair Value Measurement

The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. FASB ASC Topic 820, Fair Value Measurement and Disclosures, (“ASC 820”) established a hierarchy of inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the observable inputs be used when available. Observable inputs are inputs that market participants would use in pricing the financial instrument based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the financial instrument and are developed based on the best information available in the circumstances. The fair value hierarchy applies only to the valuation inputs used in determining the reported or disclosed fair value of the financial instruments and is not a measure of the investment credit quality. Fair value measurements are classified and disclosed in one of the following three categories:


•	Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date.


•	Level 2 utilizes quoted market prices in markets that are not active, broker or dealer quotations, or alternative pricing sources with reasonable levels of price transparency.


•	Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date.

To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

Financial instruments measured at fair value on a recurring basis include cash equivalents and investments (Note 4).

An entity may elect to measure many financial instruments and certain other items at fair value at specified election dates. Subsequent unrealized gains and losses on items for which the fair value option has been elected will be reported in net loss. The Company did not elect to measure any additional financial instruments or other items at fair value.

Revenue Recognition

Pieris has entered into several licensing agreements with collaboration partners for the development of Anticalin® therapeutics against a variety of targets in diseases and conditions. The terms of these agreements contain multiple elements and deliverables, which may include: (i) licenses, or options to obtain licenses, to Pieris’ Anticalin technology and/or specific programs and (ii) research and development activities to be performed on behalf of or with the collaborative partner. Payments to Pieris under these agreements may include upfront fees (which include license and option fees), payments for research and development activities, payments based upon the achievement of certain milestones, and royalties on product sales. There are no performance, cancellation, termination or refund provisions in any of the arrangements that could result in material financial consequences to Pieris. Pieris follows the provisions of the FASB ASC Topic 605-25, Revenue Recognition—Multiple-Element Arrangements ("ASC 605-25") and FASB ASC Topic 605-28, Revenue Recognition—Milestone Method ("ASC 605-28") in accounting for these agreements.

Multiple-Element Arrangements

When evaluating multiple-element arrangements, Pieris identifies the deliverables included within the agreement and evaluates which deliverables represent separate units of accounting based on whether the delivered element has stand-alone value to the customer or if the arrangement includes a general right of return for delivered items.

The consideration received is allocated among the separate units of accounting using the relative selling price method, and the applicable revenue recognition criteria are applied to each of the separate units of accounting. Pieris uses the best estimate of selling price (“BESP”) methodology to estimate the selling price for each deliverable and unit of accounting because Pieris does not have vendor specific objective evidence (“VSOE”) or third-party evidence (“TPE”) of selling price for these deliverables. To determine the estimated selling price of a deliverable, Pieris considers market conditions as well as entity-specific factors, including those factors contemplated in negotiating the agreements, terms of previous collaborative agreements, similar agreements entered into by third parties, market opportunity, estimated development costs, probability of success, and the time needed to commercialize a product candidate pursuant to the license. In validating Pieris’ BESP, Pieris evaluates whether changes in the key assumptions used to determine the BESP will have a significant effect on the allocation of arrangement consideration among multiple deliverables.

Multiple element arrangements, such as license arrangements, are analyzed to determine whether the deliverables, which often include licenses and performance obligations such as research and development services and governance committee services, can be separated or whether they must be accounted for as a combined unit of accounting in accordance with U.S. GAAP. The Company recognizes the arrangement consideration allocated to licenses as revenue upon delivery of the license only if the license has stand-alone value. If the license is considered not to have stand-alone value, the license would then be combined with other undelivered elements into a combined unit of accounting and the license payments and payments for performance obligations would be recognized as revenue when the revenue recognition criteria have been satisfied for the last deliverable within the unit of accounting. In the case of combined units of accounting that include delivered licenses and undelivered services to be provided over time, revenue would be recognized over the estimated period during which services will be provided. For units of accounting that include licenses to be delivered upon satisfactory completion of certain research services, revenue is deferred until the license is delivered and the performance obligation is satisfied.

If the Company is involved in a governance committee, as part of a multiple element arrangement, it assesses whether its involvement constitutes a performance obligation or a right to participate. When governance committee services are determined to be performance obligations, the Company determines the fair value to be allocated to this deliverable and recognize the revenue over the expected term of the development period of the products. Otherwise, the fair value for participation is combined with other research services or performance obligations and is recognized over the term which the Company expects to complete its aggregate performance obligations.

The Company recognizes arrangement consideration allocated to each unit of accounting when all revenue recognition criteria in ASC 605-25 are satisfied for that particular unit of accounting. For each unit of accounting, the Company must determine the period over which the performance obligations will be performed and revenue will be recognized. If there is no discernible pattern of performance or objectively measurable performance measures do not exist, then the Company recognizes revenue under the arrangement on a straight-line basis over the period the Company is expected to complete its performance obligations. Conversely, if the pattern of performance over which the service is provided to the customer can be determined and objectively measurable performance measures exist, then the Company recognizes revenue under the arrangement using the proportional performance method. Revenue recognized cannot exceed the amount that has been earned and has been billed or is currently billable.

Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company is expected to complete its performance obligations under an arrangement.

The accounting treatment for options granted to collaborators is dependent upon the nature of the option granted to the collaborative partner. Options are considered substantive if, at the inception of an agreement, Pieris is at risk as to whether the collaborative partner will choose to exercise the option(s) to secure additional goods or services. Factors that are considered in evaluating whether options are substantive include the overall objective of the arrangement, benefit the collaborator might obtain from the agreement without exercising the options, cost to exercise the options relative to the total upfront consideration, and additional financial commitments or economic penalties imposed on the collaborator as a result of exercising the options.

In arrangements where options to obtain additional deliverables are considered substantive, Pieris determines whether the optional licenses are priced at a significant and incremental discount. If the prices include a significant and incremental discount, the option is considered a deliverable in the arrangement. However, if not priced at a significant and incremental discount, the option is not considered a deliverable in the arrangement. When a collaborator exercises an option considered to be at a significant and incremental discount to acquire an additional license, the exercise fee that is attributed to the additional license and any incremental discount allocated at inception are recognized in a manner consistent with the treatment of up-front payments for licenses (i.e., license and research services). In the event an option expires un-exercised, any incremental discounts deferred at the inception of the arrangement are recognized into revenue upon expiration. For options that are non-substantive, the additional licenses to which the options pertain are considered deliverables upon inception of the arrangement; Pieris applies the multiple-element revenue recognition criteria to determine accounting treatment.

Payments or reimbursements resulting from Pieris’ research and development efforts in multi-element arrangements, in which Pieris’ research and development efforts are considered to be a deliverable, are included in allocable consideration and allocated to the units of accounting. These reimbursements are recognized as the services are performed and are presented on a gross basis, so long as there is persuasive evidence of an arrangement, the fee is fixed or determinable, and collection of the related receivable is reasonably assured. Revenue recognized cannot exceed the amount that has been earned and has been billed or is currently billable. Amounts received prior to satisfying the above revenue recognition criteria are recorded as deferred revenue in the accompanying balance sheets.

Milestone Payments and Royalties

At the inception of each agreement that includes milestone payments, Pieris evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether: (a) the consideration is commensurate with either (1) the entity’s performance to achieve the milestone, or (2) the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the entity’s performance to achieve the milestone, (b) the consideration relates solely to past performance and (c) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. Pieris evaluates factors such as the scientific, regulatory, commercial and other risks that must be overcome to achieve the respective milestone, the level of effort and investment required to achieve the respective milestone and whether the milestone consideration is reasonable relative to all deliverables and payment terms in the arrangement in making this assessment.

Pieris aggregates milestones into four categories (i) research milestones, (ii) development milestones, (iii) commercial milestones and (iv) sales milestones. Research milestones are typically achieved upon reaching certain success criteria as defined in each agreement related to developing an Anticalin protein against the specified target. Development milestones are typically reached when a compound reaches a defined phase of clinical research or passes such phase, or upon gaining regulatory approvals. Commercial milestones are typically achieved when an approved pharmaceutical product reaches the status for commercial sale, including regulatory approval. Sales milestones are certain defined levels of net sales by the licensee, such as when a product first achieves global sales or annual sales of a specified amount.

For revenues from research, development, and commercial milestone payments, if the milestones are deemed substantive and the milestone payments are nonrefundable, such amounts are recognized entirely upon successful accomplishment of the milestones, assuming all other revenue recognition criteria are met. Milestones that are not considered substantive are accounted for as contingent revenue and will be recognized when achieved to the extent the Company has no remaining performance obligations under the arrangement. Revenues from sales milestone payments are accounted for as royalties and are recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met. Royalty payments are recognized in revenues based on the timing of royalty payments earned in accordance with the agreements, which typically is the period when the relevant sales occur, assuming all other revenue recognition criteria are met.

Recent Accounting Pronouncements

In May 2014, the FASB issued Accounting Standard Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”). Subsequently, the FASB also issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606) ("ASU 2015-14"), which adjusted the effective date of ASU 2014-09; ASU No. 2016-08, Revenue from Contracts with Customers (Topic 606) ("ASU 2016-08"): Principal versus Agent Considerations (Reporting Revenue Gross versus Net), which amends the principal-versus-agent implementation guidance and illustrations in ASU 2014-09; ASU No. 2016-10, Revenue from Contracts with Customers (Topic 606) ("ASU 2016-10"): Identifying Performance Obligations and Licensing, which clarifies identifying performance obligation and licensing implementation guidance and illustrations in ASU 2014-09; and ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606) ("ASU 2016-12"): Narrow-Scope Improvements and Practical Expedients, which addresses implementation issues and is intended to reduce the cost and complexity of applying the new revenue standard in ASU 2014-09 (collectively, the “Revenue ASUs”).

The Revenue ASUs provide an accounting standard for a single comprehensive model for use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance. The accounting standard is effective for public emerging growth companies ("EGC"), like Pieris, for interim and annual periods beginning after December 15, 2018, with an option to early adopt for interim and annual periods beginning after December 15, 2017. The guidance permits two methods of adoption: retrospectively to each prior reporting period presented (the full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (the modified retrospective method). The Company is currently evaluating the impact of this pronouncement on its consolidated financial statements and expects to adopt this pronouncement commencing in the first quarter of 2019.

In February 2016, the FASB issued ASU No. 2016-2, Leases (Topic 842) ("ASU 2016-2"). Under the amendments in ASU 2016-2, lessees will be required to recognize (i) a lease liability, which is a lessee's obligation to make lease payments arising from a lease, measured on a discounted basis; and (ii) a right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term for all leases (with the exception of short-term leases) at the commencement date. This guidance is effective for fiscal years beginning after December 15, 2019 including interim periods within those fiscal years; early adoption is permitted. The Company is currently evaluating the potential impact the adoption of this standard will have on its financial statements and related disclosures.

In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act, or SAB 118, which allows the recording of provisional amounts during a measurement period not to extend beyond one year of the enactment date. In accordance with SAB 118, we determined that our deferred tax asset value and associated valuation allowance reduction of $3.7 million is a provisional amount and a reasonable estimate at December 31, 2017. The final impact may differ from this provisional amount due to, among other things, changes in interpretations and assumptions we have made thus far and the issuance of additional regulatory or other guidance. We expect to complete the final impact within the measurement period.

Pieris has considered other recent accounting pronouncements and concluded that they are either not applicable to the business, or that the effect is not expected to be material to the unaudited condensed consolidated financial statements as a result of future adoption.

Revenue

3 Months Ended

Mar. 31, 2018

Revenue from Contract with Customer [Abstract]

Revenue

General

The Company has not generated revenue from product sales. The Company has generated revenue from (i) option, license, and collaboration agreements, which include upfront payments for licenses or options to obtain licenses, payments for research and development services and milestone payments, and (ii) government grants.

During the three months ended March 31, 2018 and 2017, respectively, the Company recognized revenues as follows (in thousands):



	Three Months Ended March 31,
	2018		2017
License fees	$	3,444	$	1,008
Research and development services	617		335
Other revenues	91		—
Total Revenue	$	4,152	$	1,343

During the three months ended March 31, 2018 and 2017, respectively, the Company recognized revenue from the following strategic partnerships and other license agreements (in thousands):



	Three Months Ended March 31,
	2018		2017
Seattle Genetics	$	308	$	—
AstraZeneca	2,545		—
Servier	1,208		326
Other	91		1,017
Total Revenue	$	4,152	$	1,343

Under the Company´s existing strategic partnerships and other license agreements, Pieris could receive the following potential milestone payments (in millions):



	Research, Development & Commercial Milestones		Sales Milestones
Seattle Genetics	$	769	$	450
AstraZeneca	1,111		960
Servier	1,069		967
Other	353		126
Total potential milestone payments	$	3,302	$	2,503

Strategic Partnership with Seattle Genetics, Inc.

On February 8, 2018, the Company entered into a License and Collaboration Agreement and a Non-Exclusive Anticalin Platform Technology License Agreement (collectively the “Seattle Genetics Agreements”) with Seattle Genetics, Inc. ("Seattle Genetics"), pursuant to which the parties will develop multiple targeted bispecific immuno-oncology treatments for solid tumors and blood cancers.

Under the terms of the Seattle Genetics Agreements, the companies will pursue multiple antibody-Anticalin fusion proteins during the research phase. The Seattle Genetics Agreements provides Seattle Genetics a base option to select up to three programs for further development. Prior to the initiation of a pivotal trial, Pieris may opt into global co-development and U.S. commercialization of the second program and share in global costs and profits on an equal basis. Seattle Genetics will solely develop, fund and commercialize the other two programs. In addition, Seattle Genetics will have an option to select up to three additional programs for further development.

The Non-Exclusive Anticalin Platform Technology License Agreement (“Seattle Genetics Platform License") grants Seattle Genetics a non-exclusive license to certain intellectual property related to the Anticalin platform technology.

Upon signing the Seattle Genetics Agreements, Seattle Genetics paid Pieris a $30.0 million upfront fee and an additional $4.9 million is estimated to be paid for research and development services as reimbursement to Pieris through the end of the research term. In addition, Pieris may receive tiered royalties on net sales up to the low double-digits and up to $1.2 billion in total success-based research, development, commercial, and sales milestones payments across the product candidates, depending on the successful development and commercialization of those candidates. If Seattle Genetics exercises its option to select additional candidates from the initial research phase for further development, payment to Pieris of additional fees, milestone payments, and royalties would result.

The term of each of the Seattle Genetics Agreements ends upon the expiration of all of Seattle Genetics’ payment obligations. The License and Collaboration Agreement may be terminated by Seattle Genetics on a product-by-product basis for convenience beginning 12 months after its effective date upon 90 days' notice or, for any program where a pivotal study has been initiated, upon 180 days' notice. If any program is terminated by Seattle Genetics after a pre-defined pre-clinical stage, Pieris will have full rights to continue such program. If any program is terminated by Seattle Genetics prior to such pre-defined pre-clinical stage, Pieris will have the right to continue to develop such program, but will be obligated to offer a co-development option to Seattle Genetics for such program. The License and Collaboration Agreement may also be terminated by Seattle Genetics or Pieris for an uncured material breach by the other party upon 90 days' notice, subject to extension for an additional 90 days if the material breach relates to diligence obligations and subject, in all cases, to dispute resolution procedures. The License and Collaboration Agreement may also be terminated due to the other party’s insolvency and may in certain instances, including for reasons of safety, be terminated on a product-byproduct basis. Each party may also terminate the Seattle Genetics Agreements if the other party challenges the validity of any patents licensed under the Seattle Genetics Agreements, subject to certain exceptions. The Seattle Genetics Platform License will terminate upon termination of the License and Collaboration Agreement, whether in its entirety or on a product-by-product basis.

The Company accounted for the Seattle Genetics Agreements as a multiple element arrangement under ASC 605-25. The arrangement with Seattle Genetics contains the following initial deliverables: (i) three candidate research licenses that each consist of a non-exclusive platform technology license, a co-exclusive candidate research license, and research and development services, (ii) research, development and manufacturing services associated with each candidate research license, (iii) participation on various governance committees, and (iv) two antibody target swap options.

Management considered whether any of the deliverables could be considered separate units of accounting. The Company determined each license granted, at arrangement inception, did not have standalone value from the research and development services to be provided for the related antibody target programs due to the specific nature of the intellectual property and knowledge required to perform the research and development services. The Company determined that the participation on the various governance committees did have standalone value from the delivered licenses as the services could be performed by an outside party.

As a result, management concluded there are six units of accounting at inception of the agreement: (i) three combined units of accounting each representing a non-exclusive platform technology license, a co-exclusive candidate research license, and research and development services for first three approved Seattle Genetics antibody target programs, (ii) two units of accounting each representing an antibody target swap right for first and the second approved Seattle Genetics antibody target, and (iii) one unit of accounting representing the participation of the various governance committees.

The Company determined that neither VSOE nor TPE is available for any of the units of accounting identified at arrangement inception. Accordingly, the selling price of each unit of accounting was developed using BESP. The Company developed its best estimate of selling price for licenses by applying a risk adjusted, net present value, estimate of future potential cash flows approach, which included the cost of obtaining research and development services at arm’s length from a third-party provider, as well as internal full time equivalent costs to support these services.

The Company developed the BESP for committee participation by using management’s best estimate of the anticipated participation hours multiplied by a market rate for comparable participants.

Allocable arrangement consideration at inception is comprised of the upfront fees of $30.0 million and $4.9 million of estimated research and development services to be reimbursed as research and development occurs through the research term. Therefore, the total allocable arrangement consideration at inception is $34.9 million and is allocated among the separate units of accounting using the relative selling price method.

The amounts allocated to the combined units of accounting for the three research programs will be recognized on a proportional performance basis through the completion of each respective estimated research term for the individual research programs. However, for the first two programs that contain antibody target swap rights, if the antibody target swap right is exercised, any remaining deferred revenue associated with the two programs at the time of the exercise would be recognized immediately. The amounts allocated to the antibody target swap rights will be recognized either at the time the target right expires, or if exercised, on a proportional performance basis over the estimate research term for that program. The amounts allocated to the participation on each of the committees will be recognized ratably over the anticipated research term for all research programs.

Management determined that all research, development, commercial and sales milestones are deemed non-substantive as they are based solely on the performance of another party. Non-substantive milestones will be treated as contingent revenue and will be recognized when achieved, to the extent the Company has no remaining performance obligations under the arrangement. Milestone payments earned upon the achievement of sales events will be recognized when earned.

The Company will recognize royalty revenue in the period of sale for the related product(s), based on the underlying contract terms, provided that the reported sales are reliably measurable and the Company has no remaining performance obligations, assuming all other revenue recognition criteria are met.

As of March 31, 2018, there is $11.4 million and $18.4 million of deferred revenue and non-current deferred revenue, respectively, related to the Seattle Genetics Agreements.

Cash, Cash Equivalents and Investments

3 Months Ended

Mar. 31, 2018

Cash and Cash Equivalents [Abstract]

Cash, cash equivalents and investments

As of March 31, 2018 and December 31, 2017, cash, cash equivalents, and investments comprised of funds in depository, money market accounts, U.S. treasury securities, asset backed securities, and corporate bonds. The following table presents the cash equivalents and investments carried at fair value in accordance with the hierarchy defined in Note 2 (in thousands):



	Total		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
March 31, 2018
Money market funds, included in cash equivalents	$	67,752	$	67,752	$	—	$	—
U.S. treasuries, included in cash equivalents	6,198		6,198		—		—
Corporate bonds, included in cash equivalents	18,991		—		18,991		—
Asset-backed securities, included in cash equivalents	899		—		899		—
Investments - U.S. treasuries	4,175		4,175		—		—
Investments - Asset-backed securities	7,310		—		7,310		—
Investments - Corporate bonds	41,158		—		41,158		—
Total	$	146,483	$	78,125	$	68,358	$	—

December 31, 2017
Money market funds, included in cash equivalents	$	4,583	$	4,583	$	—	$	—
Corporate bonds, included in cash equivalents	13,595		—		13,595		—
Investments - U.S. treasuries	4,172		4,172		—		—
Investments - Asset-backed securities	6,384		—		6,384		—
Investments - Corporate bonds	34,117		—		34,117		—
Total	$	62,851	$	8,755	$	54,096	$	—

Cash equivalents and marketable securities have been initially valued at the transaction price and subsequently valued, at the end of each reporting period, utilizing third party pricing services or other market observable data. The pricing services utilize industry standard valuation models, including both income and market based approaches and observable market inputs to determine value. The Company validates the prices provided by its third-party pricing services by reviewing their pricing methods and obtaining market values from other pricing sources, as needed. After completing its validation procedures, the Company did not adjust any fair value measurements provided by the pricing services as of March 31, 2018.

Cash equivalents and investments at March 31, 2018 consist of the following (in thousands):



	Contractual maturity (in days)	Amortized Cost		Unrealized gains		Unrealized losses			Fair Value
Investments
U.S. treasuries	122-250	$	4,373	$	—	$	(198	)	$	4,175
Asset-backed securities	107-292	5,682		—		(159		)	5,523
Asset-backed securities	greater than 365	1,879		—		(92		)	1,787
Corporate bonds	25-356	41,257		13		(908		)	40,362
Corporate bonds	greater than 365	802		—		(6		)	796
Total		$	53,993	$	13	$	(1,363	)	$	52,643

The Company recorded $0.2 million of realized losses from the maturity of available-for-sale securities during the three months ended March 31, 2018. No realized gains or losses were recognized during the three months ended March 31, 2017.

Property and Equipment, Net

3 Months Ended

Mar. 31, 2018

Property, Plant and Equipment [Abstract]

Property and equipment, net

Property and equipment are summarized as follows (in thousands):



	March 31,			December 31,
	2018			2017
Laboratory equipment	$	6,656		$	6,101
Office and computer equipment	670			494
Leasehold improvements	326			318
Property and equipment at cost	7,652			6,913
Accumulated depreciation	(3,207		)	(2,879		)
Property and equipment, net	$	4,445		$	4,034

Accrued Expenses

3 Months Ended

Mar. 31, 2018

Payables and Accruals [Abstract]

Accrued Expenses

Accrued expenses and other current liabilities consisted of the following (in thousands):



	March 31,		December 31,
	2018		2017
Compensation expense	$	1,210	$	2,325
Professional fees	1,663		1,322
Research and development fees	778		791
Audit and tax fees	402		424
Other current liabilities	2,525		1,308
Total	$	6,578	$	6,170

Stockholders' Equity	3 Months Ended
Stockholders' Equity	Mar. 31, 2018
Equity [Abstract]
Stockholders' Equity	Stockholders´ Equity The Company had 53,974,184 shares of common stock and 2,907 shares of preferred stock outstanding as of March 31, 2018, both with a par value of $0.001 per share. During the three months ended March 31, 2018, 2,056 shares of preferred stock were converted into 2,056,000 shares of common stock. During the three months ended March 31, 2018 the Company issued 512,960 shares of common stock upon option exercises, resulting in cash proceeds of $0.8 million. No options were exercised during the corresponding 2017 period. During the three months ended March 31, 2018 the Company issued 63,162 shares of common stock upon exercise of warrants, resulting in cash proceeds of $0.1 million. No warrants were exercised during the corresponding 2017 period. Underwritten Public Offering In February 2018, the Company completed an underwritten public offering of its common stock in which it sold 6,325,000 shares of common stock, including the exercise in full by the underwriters of their option to purchase an additional 825,000 shares of common stock, to the public at a price of $8.00 per share (the "2018 Offering"). The 2018 Offering was completed under the Company's shelf registration statement that was filed on Form S-3 and declared effective by the SEC on August 3, 2016. Net proceeds of the 2018 Offering, after deducting the underwriting discounts and commissions, were $47.6 million, excluding offering expenses of approximately $0.4 million incurred by the Company.

Net Loss per Share	3 Months Ended
Net Loss per Share	Mar. 31, 2018
Earnings Per Share [Abstract]
Net Loss per Share	Net Loss per Share Basic net loss per share is calculated by dividing net income (loss) by the weighted average shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by adjusting weighted average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock and if-converted methods. For purposes of the diluted net loss per share calculation, preferred stock, stock options, and warrants are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, basic and diluted net loss per share were the same for all periods presented. For the three months ended March 31, 2018 and 2017, and as calculated using the treasury stock method, approximately 14.7 million and 11.2 million of weighted average shares, respectively, were excluded from the calculation of diluted weighted average shares outstanding as their effect was anti-dilutive.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Cash, Cash Equivalents and Investments

Concentration of Credit Risk and Off-Balance Sheet Risk

Fair Value Measurement


•	Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date.


•	Level 2 utilizes quoted market prices in markets that are not active, broker or dealer quotations, or alternative pricing sources with reasonable levels of price transparency.


•	Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date.

Financial instruments measured at fair value on a recurring basis include cash equivalents and investments (Note 4).

Revenue Recognition

Multiple-Element Arrangements

Milestone Payments and Royalties

Recent Accounting Pronouncements

Revenue (Tables)

3 Months Ended

Mar. 31, 2018

Revenue from Contract with Customer [Abstract]

Revenue

During the three months ended March 31, 2018 and 2017, respectively, the Company recognized revenues as follows (in thousands):



	Three Months Ended March 31,
	2018		2017
License fees	$	3,444	$	1,008
Research and development services	617		335
Other revenues	91		—
Total Revenue	$	4,152	$	1,343

During the three months ended March 31, 2018 and 2017, respectively, the Company recognized revenue from the following strategic partnerships and other license agreements (in thousands):



	Three Months Ended March 31,
	2018		2017
Seattle Genetics	$	308	$	—
AstraZeneca	2,545		—
Servier	1,208		326
Other	91		1,017
Total Revenue	$	4,152	$	1,343

Potential Milestone Payments

Under the Company´s existing strategic partnerships and other license agreements, Pieris could receive the following potential milestone payments (in millions):



	Research, Development & Commercial Milestones		Sales Milestones
Seattle Genetics	$	769	$	450
AstraZeneca	1,111		960
Servier	1,069		967
Other	353		126
Total potential milestone payments	$	3,302	$	2,503

Cash, Cash Equivalents and Investments (Tables)

3 Months Ended

Mar. 31, 2018

Cash and Cash Equivalents [Abstract]

Cash Equivalents and Investments Carried at Fair Value

The following table presents the cash equivalents and investments carried at fair value in accordance with the hierarchy defined in Note 2 (in thousands):



	Total		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
March 31, 2018
Money market funds, included in cash equivalents	$	67,752	$	67,752	$	—	$	—
U.S. treasuries, included in cash equivalents	6,198		6,198		—		—
Corporate bonds, included in cash equivalents	18,991		—		18,991		—
Asset-backed securities, included in cash equivalents	899		—		899		—
Investments - U.S. treasuries	4,175		4,175		—		—
Investments - Asset-backed securities	7,310		—		7,310		—
Investments - Corporate bonds	41,158		—		41,158		—
Total	$	146,483	$	78,125	$	68,358	$	—

December 31, 2017
Money market funds, included in cash equivalents	$	4,583	$	4,583	$	—	$	—
Corporate bonds, included in cash equivalents	13,595		—		13,595		—
Investments - U.S. treasuries	4,172		4,172		—		—
Investments - Asset-backed securities	6,384		—		6,384		—
Investments - Corporate bonds	34,117		—		34,117		—
Total	$	62,851	$	8,755	$	54,096	$	—

Cash Equivalents and Investments

Cash equivalents and investments at March 31, 2018 consist of the following (in thousands):



	Contractual maturity (in days)	Amortized Cost		Unrealized gains		Unrealized losses			Fair Value
Investments
U.S. treasuries	122-250	$	4,373	$	—	$	(198	)	$	4,175
Asset-backed securities	107-292	5,682		—		(159		)	5,523
Asset-backed securities	greater than 365	1,879		—		(92		)	1,787
Corporate bonds	25-356	41,257		13		(908		)	40,362
Corporate bonds	greater than 365	802		—		(6		)	796
Total		$	53,993	$	13	$	(1,363	)	$	52,643

Property and Equipment, Net (Tables)

3 Months Ended

Mar. 31, 2018

Property, Plant and Equipment [Abstract]

Summary of Property and Equipment

Property and equipment are summarized as follows (in thousands):



	March 31,			December 31,
	2018			2017
Laboratory equipment	$	6,656		$	6,101
Office and computer equipment	670			494
Leasehold improvements	326			318
Property and equipment at cost	7,652			6,913
Accumulated depreciation	(3,207		)	(2,879		)
Property and equipment, net	$	4,445		$	4,034

Accrued Expenses (Tables)

3 Months Ended

Mar. 31, 2018

Payables and Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):



	March 31,		December 31,
	2018		2017
Compensation expense	$	1,210	$	2,325
Professional fees	1,663		1,322
Research and development fees	778		791
Audit and tax fees	402		424
Other current liabilities	2,525		1,308
Total	$	6,578	$	6,170

Corporate Information (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2014
Business Acquisition [Line Items]
Cash, cash equivalents and investments	$ 162.2
Pieris Pharmaceuticals GmbH
Business Acquisition [Line Items]
Ownership interest acquired		100.00%

Summary of Significant Accounting Policies (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Accounting Policies [Abstract]
Realized losses recognized on available-for-sale investments	$ 200,000
Realized gains (losses) recognized on available-for-sale investments		$ 0
Effect of remeasurement of deferred tax assets and liabilities under Tax Cuts and Jobs Act			$ 3,700,000

Revenue - Revenue Generated from Non-Product Sales (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue from Contract with Customer [Abstract]
License fees	$ 3,444	$ 1,008
Research and development services	617	335
Other revenues	91	0
Total Revenue	$ 4,152	$ 1,343

Revenue - Revenue From Licensing Agreements and Strategic Partnerships (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 4,152	$ 1,343
Seattle Genetics
Disaggregation of Revenue [Line Items]
Revenue	308	0
AstraZeneca
Disaggregation of Revenue [Line Items]
Revenue	2,545	0
Servier
Disaggregation of Revenue [Line Items]
Revenue	1,208	326
Other
Disaggregation of Revenue [Line Items]
Revenue	$ 91	$ 1,017

Revenue - Potential Milestone Payments (Details) - Strategic Partnerships and Other License Agreements $ in Millions	Mar. 31, 2018 USD ($)
Research, Development & Commercial Milestones
Revenue Recognition, Milestone Method [Line Items]
Potential milestone payments	$ 3,302
Sales Milestones
Revenue Recognition, Milestone Method [Line Items]
Potential milestone payments	2,503
Seattle Genetics \| Research, Development & Commercial Milestones
Revenue Recognition, Milestone Method [Line Items]
Potential milestone payments	769
Seattle Genetics \| Sales Milestones
Revenue Recognition, Milestone Method [Line Items]
Potential milestone payments	450
AstraZeneca \| Research, Development & Commercial Milestones
Revenue Recognition, Milestone Method [Line Items]
Potential milestone payments	1,111
AstraZeneca \| Sales Milestones
Revenue Recognition, Milestone Method [Line Items]
Potential milestone payments	960
Servier \| Research, Development & Commercial Milestones
Revenue Recognition, Milestone Method [Line Items]
Potential milestone payments	1,069
Servier \| Sales Milestones
Revenue Recognition, Milestone Method [Line Items]
Potential milestone payments	967
Other \| Research, Development & Commercial Milestones
Revenue Recognition, Milestone Method [Line Items]
Potential milestone payments	353
Other \| Sales Milestones
Revenue Recognition, Milestone Method [Line Items]
Potential milestone payments	$ 126

Revenue - Seattle Genetics, Inc. (Details) - USD ($)	Feb. 08, 2018	Mar. 31, 2018	Dec. 31, 2017
Deferred Revenue Arrangement [Line Items]
Deferred revenues, current portion		$ 46,251,000	$ 37,153,000
Non-current deferred revenue		67,746,000	$ 46,542,000
Seattle Genetics \| License and Collaboration Agreement
Deferred Revenue Arrangement [Line Items]
Upfront fees received	$ 30,000,000
Estimated reimbursements for research, development and manufacturing services	4,900,000
Maximum tiered royalties on net sales	$ 1,200,000,000
Period after effective date agreements may be terminated	12 months
Advance notice period for contract termination	90 days
Allocable arrangement consideration	$ 34,900,000
Deferred revenues, current portion		11,400,000
Non-current deferred revenue		$ 18,400,000
Seattle Genetics \| License and Collaboration Agreement \| Program Pivotal Study Initiated
Deferred Revenue Arrangement [Line Items]
Advance notice period for contract termination	180 days
Seattle Genetics \| License and Collaboration Agreement \| Material Breach of Contract
Deferred Revenue Arrangement [Line Items]
Advance notice period for contract termination	90 days
Seattle Genetics \| License and Collaboration Agreement \| Material Breach of Contract due to Diligence Obligations
Deferred Revenue Arrangement [Line Items]
Advance notice period for contract termination	90 days

Cash, Cash Equivalents and Investments - Cash Equivalents and Investments Carried at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 146,483	$ 62,851
Quoted prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	78,125	8,755
Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	68,358	54,096
Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
U.S. treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	4,175	4,172
U.S. treasuries \| Quoted prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	4,175	4,172
U.S. treasuries \| Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
U.S. treasuries \| Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	7,310	6,384
Asset-backed securities \| Quoted prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Asset-backed securities \| Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	7,310	6,384
Asset-backed securities \| Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	41,158	34,117
Corporate bonds \| Quoted prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Corporate bonds \| Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	41,158	34,117
Corporate bonds \| Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	67,752	4,583
Money market funds \| Quoted prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	67,752	4,583
Money market funds \| Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Money market funds \| Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
U.S. treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	6,198
U.S. treasuries \| Quoted prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	6,198
U.S. treasuries \| Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
U.S. treasuries \| Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	18,991	13,595
Corporate bonds \| Quoted prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Corporate bonds \| Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	18,991	13,595
Corporate bonds \| Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	$ 0
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	899
Asset-backed securities \| Quoted prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Asset-backed securities \| Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	899
Asset-backed securities \| Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 0

Cash, Cash Equivalents and Investments - Cash Equivalents and Investments (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 53,993
Unrealized gains	13
Unrealized losses	(1,363)
Fair Value	52,643
U.S. treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	4,373
Unrealized gains	0
Unrealized losses	(198)
Fair Value	$ 4,175
U.S. treasuries \| Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contractual maturity	122 days
U.S. treasuries \| Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contractual maturity	250 days
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 5,682
Unrealized gains	0
Unrealized losses	(159)
Fair Value	$ 5,523
Asset-backed securities \| Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contractual maturity	107 days
Asset-backed securities \| Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contractual maturity	292 days
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 1,879
Unrealized gains	0
Unrealized losses	(92)
Fair Value	$ 1,787
Asset-backed securities \| Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contractual maturity	365 days
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 41,257
Unrealized gains	13
Unrealized losses	(908)
Fair Value	$ 40,362
Corporate bonds \| Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contractual maturity	25 days
Corporate bonds \| Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contractual maturity	356 days
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 802
Unrealized gains	0
Unrealized losses	(6)
Fair Value	$ 796
Corporate bonds \| Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contractual maturity	365 days

Cash, Cash Equivalents and Investments - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Cash and Cash Equivalents [Abstract]
Realized losses from the maturity of available-for-sale securities	$ 0.2

Property and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment at cost	$ 7,652	$ 6,913
Accumulated depreciation	(3,207)	(2,879)
Property and equipment, net	4,445	4,034
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property and equipment at cost	6,656	6,101
Office and computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment at cost	670	494
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment at cost	$ 326	$ 318

Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accrued expenses
Compensation expense	$ 1,210	$ 2,325
Professional fees	1,663	1,322
Research and development fees	778	791
Audit and tax fees	402	424
Other current liabilities	2,525	1,308
Total	$ 6,578	$ 6,170

Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Class of Stock [Line Items]
Shares of common stock outstanding (in shares)	53,974,184		45,017,062
Shares of preferred stock outstanding (in shares)	2,907		4,963
Par value of common stock (in dollars per share)	$ 0.001		$ 0.001
Par value of preferred stock (in dollars per share)	$ 0.001		$ 0.001
Cash proceeds from option exercises	$ 826	$ 0
Cash proceeds from exercise of warrants	$ 126	$ 0
Preferred Stock
Class of Stock [Line Items]
Shares of preferred stock converted to common stock (in shares)	2,056
Common Shares
Class of Stock [Line Items]
Shares of common stock issued upon conversion of preferred stock (in shares)	2,056,000
Cash proceeds from option exercises	$ 800
Cash proceeds from exercise of warrants	$ 100
Common Shares \| Options
Class of Stock [Line Items]
Shares of common stock issued upon exercises (in shares)	512,960	0
Common Shares \| Warrants
Class of Stock [Line Items]
Shares of common stock issued upon exercises (in shares)	63,162	0

Stockholders' Equity - Underwritten Public Offering (Details) $ / shares in Units, $ in Millions	1 Months Ended
	Feb. 28, 2018 USD ($) $ / shares shares
Class of Stock [Line Items]
Price per share sold (in dollars per share) \| $ / shares	$ 8.00
Stock offering costs \| $	$ 0.4
Public Stock Offering
Class of Stock [Line Items]
Shares sold in offering (in shares) \| shares	6,325,000
Net proceeds from stock offering \| $	$ 47.6
Over-Allotment Option
Class of Stock [Line Items]
Shares sold in offering (in shares) \| shares	825,000

Net Loss per Share (Details) - shares shares in Millions	3 Months Ended
Net Loss per Share (Details) - shares shares in Millions	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Weighted average shares excluded from the calculation of diluted weighted average shares outstanding (in shares)	14.7	11.2