Prothena (PRTA) 10-K 25 Feb 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Feb. 12, 2016	Jun. 30, 2015
Document Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	FY
Trading Symbol	PRTA
Entity Registrant Name	Prothena Corp plc
Entity Central Index Key	1,559,053
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Ordinary Shares Outstanding		34,338,727
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Public Float			$ 1,651.6

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 370,586	$ 293,579
Receivable from Roche	509	1,729
Receivable from related party	0	30
Prepaid expenses and other current assets	6,308	3,770
Total current assets	377,403	299,108
Non-current assets:
Property and equipment, net	3,862	3,121
Deferred tax assets	2,850	1,887
Other non-current assets	1,121	0
Total non-current assets	7,833	5,008
Total assets	385,236	304,116
Current liabilities:
Accounts payable	4,519	4,722
Accrued research and development	12,794	2,285
Income taxes payable	0	57
Other current liabilities	4,903	4,975
Total current liabilities	22,216	12,039
Non-current liabilities:
Income taxes payable, non-current	98	98
Deferred rent	2,127	2,090
Other liabilities	126	0
Total non-current liabilities	2,351	2,188
Total liabilities	$ 24,567	$ 14,227
Commitments and contingencies (Note 6)
Shareholders’ equity:
Euro deferred shares, €22 nominal value: Authorized shares - 10,000 at December 31, 2015 and December 31, 2014 Issued and outstanding shares - none at December 31, 2015 and 2014	$ 0	$ 0
Ordinary shares, $0.01 par value: Authorized shares - 100,000,000 at December 31, 2015 and 2014 Issued and outstanding shares - 31,744,102 and 27,388,005 at December 31, 2015 and 2014, respectively	317	274
Additional paid-in capital	489,455	338,106
Accumulated deficit	(129,103)	(48,491)
Total shareholders’ equity	360,669	289,889
Total liabilities and shareholders’ equity	$ 385,236	$ 304,116

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical)	Dec. 31, 2015$ / sharesshares	Dec. 31, 2015€ / sharesshares	Dec. 31, 2014$ / sharesshares	Dec. 31, 2014€ / sharesshares
Statement of Financial Position [Abstract]
Euro deferred shares, nominal value (in euros per share) | € / shares		€ 22		€ 22
Euro deferred shares, number of shares authorized	10,000	10,000	10,000	10,000
Euro deferred shares, number of issued shares	0	0	0	0
Euro deferred shares, number of outstanding shares	0	0	0	0
Ordinary shares, par value (in dollars per share) | $ / shares	$ 0.01		$ 0.01
Ordinary shares, number of authorized shares	100,000,000	100,000,000	100,000,000	100,000,000
Ordinary shares, number of issued shares	31,744,102	31,744,102	27,388,005	27,388,005
Ordinary shares, number of outstanding shares	31,744,102	31,744,102	27,388,005	27,388,005

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Collaboration revenue	$ 1,607	$ 50,320	$ 0
Revenue—related party	0	534	676
Total revenue	1,607	50,854	676
Operating expenses:
Research and development	58,439	38,452	26,052
General and administrative	23,105	19,051	15,051
Total operating expenses	81,544	57,503	41,103
Loss from operations	(79,937)	(6,649)	(40,427)
Other income (expense):
Interest income	196	79	71
Other income (expense), net	(170)	231	(225)
Total other income (expense)	26	310	(154)
Loss before income taxes	(79,911)	(6,339)	(40,581)
Provision for income taxes	701	811	415
Net loss	$ (80,612)	$ (7,150)	$ (40,996)
Basic and diluted net loss per share (in dollars per share)	$ (2.66)	$ (0.29)	$ (2.20)
Shares used to compute basic and diluted net loss per share (in shares)	30,326	24,672	18,615

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net loss	$ (80,612)	$ (7,150)	$ (40,996)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	806	743	660
Share-based compensation	10,414	5,597	3,128
Excess tax benefit from share-based award exercises	(3,855)	(242)	0
Deferred income taxes	(963)	(990)	(538)
(Gain) loss on disposal of fixed asset	20	(19)	(29)
Loss on sublease	261	0	0
Changes in operating assets and liabilities:
Receivable from Roche	1,220	(1,729)	0
Receivable from related party	30	28	165
Other assets	(937)	(2,352)	(721)
Accounts payable, accruals and other liabilities	11,161	5,431	6,233
Net cash used in operating activities	(62,455)	(683)	(32,098)
Investing activities
Purchases of property and equipment	(1,382)	(526)	(564)
Proceeds from disposal of fixed asset	0	27	29
Net cash used in investing activities	(1,382)	(499)	(535)
Financing activities
Proceeds from issuance of ordinary shares in public offering, net	131,341	117,348	84,534
Post separation adjustments to the funding provided by Elan	0	0	(84)
Proceeds from issuance of ordinary shares upon exercise of stock options	5,648	494	0
Excess tax benefit from share-based award exercises	3,855	242	0
Net cash provided by financing activities	140,844	118,084	84,450
Net increase in cash and cash equivalents	77,007	116,902	51,817
Cash and cash equivalents, beginning of the year	293,579	176,677	124,860
Cash and cash equivalents, end of the period	370,586	293,579	176,677
Supplemental disclosures of cash flow information
Cash paid for income taxes, net of refunds	442	1,588	796
Supplemental disclosures of non-cash investing and financing activities
Acquisition of property and equipment included in accounts payable and accrued liabilities	185	0	26
Offering costs included in accounts payable and accrued liabilities	18	0	82
Receivable from stock option exercises	$ 0	$ 6	$ 0

Consolidated Statement of Share

Consolidated Statement of Shareholders' Equity Statement - USD ($) $ in Thousands	Total	Ordinary Shares	Additional Paid-in Capital	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2012		17,679,182
Beginning balance, value at Dec. 31, 2012	$ 126,484	$ 177	$ 126,652	$ (345)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of ordinary shares in public offering, net of issuance costs (in shares)		4,177,079
Issuance of ordinary shares in public offering, net of issuance cost	84,453	$ 42	84,411
Issuance costs	7,400
Share-based compensation	3,128		3,128
Post separation adjustment to the funding provided by Elan	201		201
Excess tax benefit from share-based award exercises	0
Net loss	(40,996)			(40,996)
Ending balance (in shares) at Dec. 31, 2013		21,856,261
Ending balance, value at Dec. 31, 2013	173,270	$ 219	214,392	(41,341)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of ordinary shares in public offering, net of issuance costs (in shares)		5,462,500
Issuance of ordinary shares in public offering, net of issuance cost	117,429	$ 54	117,375
Issuance costs	5,500
Share-based compensation	5,597		5,597
Excess tax benefit from share-based award exercises	242		242
Issuance of ordinary shares upon exercise of stock options (in shares)		69,244
Issuance of ordinary shares upon exercise of stock options	501	$ 1	500
Net loss	(7,150)			(7,150)
Ending balance (in shares) at Dec. 31, 2014		27,388,005
Ending balance, value at Dec. 31, 2014	289,889	$ 274	338,106	(48,491)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of ordinary shares in public offering, net of issuance costs (in shares)		3,795,000
Issuance of ordinary shares in public offering, net of issuance cost	131,481	$ 38	131,443
Issuance costs	8,900
Share-based compensation	10,414		10,414
Excess tax benefit from share-based award exercises	3,855		3,855
Issuance of ordinary shares upon exercise of stock options (in shares)		561,097
Issuance of ordinary shares upon exercise of stock options	5,642	$ 5	5,637
Net loss	(80,612)			(80,612)
Ending balance (in shares) at Dec. 31, 2015		31,744,102
Ending balance, value at Dec. 31, 2015	$ 360,669	$ 317	$ 489,455	$ (129,103)

Organization

Organization	12 Months Ended
	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization	Organization Description of Business Prothena Corporation plc and its subsidiaries (“Prothena” or the “Company”) is a global biotechnology company seeking to fundamentally change the course of progressive diseases, with its late-stage clinical pipeline of novel therapeutic antibodies. The Company's clinical pipeline of antibody-based product candidates target a number of potential indications including AL amyloidosis (NEOD001), Parkinson’s disease and other related synucleinopathies (PRX002) and inflammatory diseases including psoriasis (PRX003). The Company is a public limited company formed under the laws of Ireland. The Company separated from Elan Corporation, plc (“Elan”) on December 20, 2012. After the separation from Elan, and the related distribution of the Company's ordinary shares to Elan’s shareholders, the Company's ordinary shares commenced trading on The Nasdaq Global Market under the symbol “PRTA” on December 21, 2012 and currently trade on The Nasdaq Global Select Market. Liquidity and Business Risks As of December 31, 2015 , the Company had an accumulated deficit of $129.1 million and cash and cash equivalents of $370.6 million . Based on the Company's business plans, management believes that the Company's cash and cash equivalents at December 31, 2015 are sufficient to meet its obligations for at least the next twelve months. To operate beyond such period, or if the Company elects to increase its spending on development programs significantly above current long-term plans or enters into potential licenses and or other acquisitions of complementary technologies, products or companies, the Company may need additional capital. The Company expects to continue to finance future cash needs that exceed its cash from operating activities primarily through its current cash and cash equivalents, its collaboration with Roche, and to the extent necessary, through proceeds from public or private equity or debt financings, loans and other collaborative agreements with corporate partners or other arrangements. The Company is subject to a number of risks, including but not limited to: the uncertainty of the Company's research and development (“R&D”) efforts resulting in future successful commercial products; obtaining regulatory approval for its product candidates; its ability to successfully commercialize its product candidates, if approved; significant competition from larger organizations; reliance on the proprietary technology of others; dependence on key personnel; uncertain patent protection; dependence on corporate partners and collaborators; and possible restrictions on reimbursement from governmental agencies and healthcare organizations, as well as other changes in the healthcare industry.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Preparation and Presentation of Financial Information These Consolidated Financial Statements have been prepared in accordance with the accounting principles generally accepted in the U.S. (“GAAP”) and with the instructions for Form 10-K and Regulations S-X statements. The Consolidated Financial Statements of Prothena Corporation plc are presented in U.S. dollars, which is the functional currency of the Company. These Consolidated Financial Statements include the accounts of the Company and its consolidated subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Certain amounts in the Consolidated Financial Statements have been reclassified to conform to the current year presentation. Use of Estimates The preparation of the Consolidated Financial Statements in conformity with GAAP requires management to make judgments, estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and related disclosures. On an ongoing basis, management evaluates its estimates, including critical accounting policies or estimates related to revenue recognition, share-based compensation and research and development expenses. The Company bases its estimates on historical experience and on various other market specific and other relevant assumptions that management believes to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Because of the uncertainties inherent in such estimates, actual results may differ materially from these estimates. Significant Accounting Policies Cash and Cash Equivalents The Company considers all highly liquid investments held at financial institutions, such as commercial paper, money market funds, and other money market securities with original maturities of three months or less at date of purchase to be cash equivalents. Property and Equipment, net Property and equipment, net are stated at cost less accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the related assets. Maintenance and repairs are charged to expense as incurred, and improvements and betterments are capitalized. When assets are retired or otherwise disposed of, the cost and accumulated depreciation are removed from the balance sheet and any resulting gain or loss is reflected in operations in the period realized. Depreciation and amortization periods for the Company’s property, plant and equipment are as follows: Useful Life Machinery and equipment 4-7 years Leasehold improvements Shorter of expected useful life or lease term Purchased computer software 4 years Impairment of Long-lived Assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable or the estimated useful life is no longer appropriate. If circumstances require that a long-lived asset be tested for possible impairment, the Company compares the undiscounted cash flows expected to be generated by the asset to the carrying amount of the asset. If the carrying amount of the long-lived asset is not recoverable on an undiscounted cash flow basis, an impairment is recognized to the extent that the carrying amount exceeds its fair value. The Company determines fair value using the income approach based on the present value of expected future cash flows. The Company’s cash flow assumptions consider historical and forecasted revenue and operating costs and other relevant factors. There were no impairment charges recorded during the years ended December 31, 2015, 2014 and 2013 . Deferred Rent Deferred rent consists of the difference between cash payments and the recognition of rent expense on a straight-line basis for the buildings the Company occupies. The leases provide for fixed increases in minimum annual rental payments, as well as rent free periods. The total amount of rental payments due over the lease terms are being charged to rent expense ratably over the life of the leases. Tenant improvement allowances are recorded as a deferred rent liability and are amortized over the term of the lease as a reduction to rent expense. Revenue Recognition Revenue is recognized when earned and non-refundable, when payment is reasonably assured, and when there is no future obligation with respect to the revenue, in accordance with the terms prescribed in the applicable contract. Multiple Element Arrangements The Company's revenues are generated primarily through its license, development and commercialization agreement. These types of agreements generally contain multiple elements, or deliverables, which may include (i) licenses to the Company's technology, (ii) R&D activities to be performed on behalf of the collaborative partner, and (iii) in certain cases, services or obligations in connection with the manufacturing or supply of preclinical and clinical material. Payments to the Company under these arrangements typically include one or more of the following: non-refundable, upfront license fees; funding of research and/or development efforts; milestone payments; and royalties on future product sales. Revenue under license, development and commercialization agreements is recognized based on the performance requirements of the contract. Determinations of whether persuasive evidence of an arrangement exists and whether delivery has occurred or services have been rendered are based on management’s judgments regarding the fixed nature of the fees charged for deliverables and the collectability of those fees. The Company recognizes revenue related to license, development and commercialization agreements in accordance with the provisions of Financial Accounting Standards Board, or FASB, Accounting Standards Codification, or ASC, Topic 605-25 , “Revenue Recognition - Multiple-Element Arrangements.” The Company evaluates all deliverables within an arrangement to determine whether or not they provide value on a stand-alone basis. Based on this evaluation, the deliverables are separated into units of accounting. The arrangement consideration that is fixed or determinable at the inception of the arrangement is allocated to the separate units of accounting based on their relative selling prices. To determine the selling price of a separate deliverable, the Company uses the hierarchy as prescribed in ASC Topic 605-25 based on vendor-specific objective evidence (VSOE), third-party evidence (TPE) or best estimate of selling price (BESP). VSOE is based on the price charged when the element is sold separately and is the price actually charged for that deliverable. TPE is determined based on third party evidence for a similar deliverable when sold separately and BESP is the estimated selling price at which we would transact a sale if the elements of collaboration and license arrangements were sold on a stand-alone basis to the buyer. Payments or full reimbursements resulting from our R&D efforts for those arrangements where such efforts are considered as deliverables 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. However, such funding is recognized as a reduction of R&D expense when the Company engages in a R&D project jointly with another entity, with both entities participating in project activities and sharing costs and potential benefits of the project. Milestone Revenue The Company accounts for milestones under ASU No. 2010-17 , Milestone Method of Revenue Recognition. Under the milestone method, contingent consideration received from the achievement of a substantive milestone is recognized in its entirety in the period in which the milestone is achieved. A milestone is defined as an event (i) that can only be achieved based in whole or in part on either the entity’s performance or on the occurrence of a specific outcome resulting from the entity’s performance, (ii) for which there is substantive uncertainty at the date the arrangement is entered into that the event will be achieved, and (iii) that would result in additional payments being due to the entity. At the inception of an agreement that includes milestone payments, the Company 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. The Company evaluates factors such as the scientific, regulatory, commercial and other risks that must be overcome to achieve a particular milestone, the level of effort and investment required to achieve such milestone and whether the milestone consideration is reasonable relative to all deliverables and payment terms in the arrangement in making this assessment. The Company generally classifies each of its milestones into one of three categories: (i) clinical milestones, (ii) regulatory and development milestones, and (iii) commercial milestones. Clinical milestones are typically achieved when a product candidate advances into or completes a defined phase of clinical research. For example, a milestone payment may be due to the Company upon the initiation of a clinical trial for a new indication. Regulatory and development milestones are typically achieved upon acceptance of the submission for marketing approval of a product candidate or upon approval to market the product candidate by the FDA or other regulatory authorities. For example, a milestone payment may be due to the Company upon filing of a Biologics License Application (BLA) with the FDA. Commercial milestones are typically achieved when an approved pharmaceutical product reaches certain defined levels of net royalty sales by the licensee of a specified amount within a specified period. Commercial milestone payments and milestone payments that are not deemed to be substantive will be accounted for as a contingent revenue payment with revenue recognized when all contingencies are lifted, which is expected to be upon achievement of the milestone, assuming all revenue recognition criteria are met. Profit Share Revenue For agreements, with profit sharing arrangements, the Company will record its share of the pre-tax commercial profit as collaboration revenue when the profit sharing can be reasonably estimated and collectability is reasonably assured. Royalty Revenue The Company will recognize revenue from royalties based on licensees' sales of the Company's products or products using the Company's technologies. Royalties are recognized as earned in accordance with the contract terms when royalties from licensees can be reasonably estimated and collectability is reasonably assured. Research and Development Research and development costs are expensed as incurred and include, but are not limited to, salary and benefits, share-based compensation, clinical trial activities, drug development and manufacturing prior to FDA approval and third-party service fees, including clinical research organizations and investigative sites. Costs for certain development activities, such as clinical trials, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, or information provided to the Company by its vendors on their actual costs incurred. The objective of the Company's accrual policy is to match the recording of the expenses in its Consolidated Financial Statements to the actual services received and efforts expended. As such, expense accruals related to clinical trials are recognized based on its estimate of the degree of completion of the events specified in the specific clinical study or trial contract. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the Consolidated Financial Statements as prepaid or accrued research and development. Amounts due may be fixed fee, fee for service, and may include upfront payments, monthly payments, and payments upon the completion of milestones or receipt of deliverables. Acquired In-Process Research and Development Expense The Company has acquired and may continue to acquire the rights to develop and commercialize new drug candidates from third parties. The upfront payments to acquire license, product or rights, as well as any future milestone payments, are immediately expensed as research and development provided that the drug has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use. Share-based Compensation To determine the fair value of share-based payment awards, the Company uses the Black-Scholes option-pricing model. The determination of fair value using the Black-Scholes option-pricing model is affected by the Company’s share price as well as assumptions regarding a number of complex and subjective variables. Share-based compensation expense is recognized on a straight-line basis over the requisite service period for each award. Further, share-based compensation expense recognized in the Consolidated Statements of Operations is based on awards expected to vest and therefore the amount of expense has been reduced for estimated forfeitures. If actual forfeitures differ from estimates at the time of grant they will be revised in subsequent periods. The Company bases its assumptions on historical data when available or when not available, on a peer group of companies. If factors change and different assumptions are employed in determining the fair value of share-based awards, the share-based compensation expense recorded in future periods may differ significantly from what was recorded in the current period (see Note 9 for further information). Income Taxes The Company files its own U.S. and foreign income tax returns and income taxes are presented in the Consolidated Financial Statements using the asset and liability method prescribed by the accounting guidance for income taxes. Deferred tax assets (“DTAs”) and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using the enacted tax rates projected to be in effect for the year in which the differences are expected to reverse. Net deferred tax assets are recorded to the extent the Company believes that these assets will more likely than not be realized. In making such determination, all available positive and negative evidence is considered, including scheduled reversals of deferred tax liabilities, projected future taxable income, tax planning strategies and recent financial operations. Estimates are required in determining the Company’s provision for income taxes. Some of these estimates are based on management’s interpretations of jurisdiction-specific tax laws or regulations. Various internal and external factors may have favorable or unfavorable effects on the future effective income tax rate of the business. These factors include, but are not limited to, changes in tax laws, regulations and/or rates, changing interpretations of existing tax laws or regulations, changes in estimates of prior years’ items, past and future levels of R&D spending and changes in overall levels of income before taxes. The tax benefit from an uncertain tax position is recognized only if it is more likely than not the tax position will be sustained on examination by the taxing authorities, based on the technical merits of the position. The tax benefits recognized in the financial statements from such positions are then measured based on the largest benefit that has a greater than 50% likelihood of being realized upon settlement. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs. Interest and penalties related to unrecognized tax benefits are accounted for in income tax expense. Net Income (loss) per Ordinary Share Basic net income (loss) per ordinary share is computed by dividing net income (loss) attributable to ordinary shareholders by the weighted average number of ordinary shares outstanding during the period. Diluted net income per ordinary share is computed by giving effect to all dilutive potential ordinary shares including options. However, potentially issuable ordinary shares are not used in computing diluted net loss per ordinary share as their effect would be anti-dilutive due to the loss recorded. In this case, diluted net loss per share is equal to basic net loss per share. Comprehensive Loss Comprehensive income (loss) is comprised of net income (loss) and other comprehensive income (loss). The Company has no components of other comprehensive income (loss). Therefore net income (loss) equals comprehensive income (loss) for all periods presented and, accordingly, the Consolidated Statements of Comprehensive Income (Loss) is not presented in a separate statement. Segment and Concentration of Risks The Company operates in one segment. The Company’s chief operating decision maker (the “CODM”), its Chief Executive Officer, manages the Company’s operations on a consolidated basis for purposes of allocating resources. When evaluating the Company’s financial performance, the CODM reviews all financial information on a consolidated basis. Financial instruments that potentially subject the Company to concentration of credit risk consist of cash and cash equivalents and accounts receivable. The Company places its cash equivalents with high credit quality financial institutions and by policy, limits the amount of credit exposure with any one financial institution. Deposits held with banks may exceed the amount of insurance provided on such deposits. The Company has not experienced any losses on its deposits of cash and cash equivalents and its credit risk exposure is up to the extent recorded on the Company's consolidated balance sheet. Receivable from Roche as of December 31, 2015 and 2014 are amounts due from Roche entities located in the U.S. and Switzerland under the License Agreement that became effective January 22, 2014. The Company’s receivable from related party as of December 31, 2014 is from Elan located in Ireland. Revenue recorded in the Statements of Operations consists of collaboration revenue related to the upfront payment from Roche under the License Agreement, payment from Roche upon achievement of a clinical milestone and reimbursement for research and development services and fees earned from the provision of nonclinical research support to Elan, primarily in the areas of safety, toxicology and regulatory. Credit risk exposure is up to the extent recorded on the Company's Consolidated Balance Sheet. As of December 31, 2015, $3.1 million of the Company's long-lived assets were held in the U.S. and $0.8 million were in Ireland. As of December 31, 2014, all of the Company's long-lived assets were held in the U.S. The Company does not own or operate facilities for the manufacture, storage, testing or distribution of preclinical or clinical supplies of any of its drug candidates. The Company instead contracted with and relies on third-parties to manufacture, store, test and distribute all preclinical development and clinical supplies of its drug candidates, and the Company plans to continue to do so for the foreseeable future. Currently, the Company has a single source of preclinical or clinical supplies for each of its drug candidates. A delay or inability to obtain such supply could have an adverse effect on the Company's business, financial condition and results of operations. Recent Accounting Pronouncements In May 2014, the FASB issued Accounting Standards Update 2014-09 (ASU 2014-09), Revenue from Contracts with Customers. ASU 2014-09 supersedes the revenue recognition requirements in Revenue Recognition (Topic 605), and requires entities to recognize revenue in a way that depicts the transfer of promised goods and services to customers in an amount that reflects the consideration to which the entity expects to be entitled to in exchange for those goods or services. ASU 2014-09 is effective for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period, which for the Company is January 1, 2018. Early adoption is permitted after January 1, 2017. The standard permits the use of either retrospective or cumulative effect transition method. The Company is currently evaluating the potential impact the adoption of ASU 2014-09 will have on its consolidated financial statements. The Company has not yet selected a transition method nor has it determined the effect of the standard on its ongoing financial reporting. In November 2015, the FASB issued an Accounting Standards Update 2015-17 (ASU 2015-17), Income Taxes (Topic 740): Balance Sheet Classification of Deferred Taxes. ASU 2015-17 requires entities with classified balance sheet to present all deferred tax assets and liabilities as noncurrent and is effective for annual and interim periods beginning after December 15, 2016. However, early adoption is permitted for any financial statements that have not yet been issued. ASU 2015-17 allows entities to choose either prospective or retrospective transition, the latter of which was chosen by the Company. In these Consolidated Financial Statements the Company has presented its net deferred tax assets as noncurrent as of December 31, 2015 and 2014. The early adoption of this standard did not have a material impact on the Company's Consolidated Financial Statements since the portion of current Deferred Tax Assets in prior year of $167,000 which was reclassified as noncurrent is not significant.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The Company measures certain financial assets and liabilities at fair value on a recurring basis, including cash equivalents. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or a liability. A three-tier fair value hierarchy is established as a basis for considering such assumptions and for inputs used in the valuation methodologies in measuring fair value: Level 1 — Observable inputs such as quoted prices (unadjusted) for identical assets or liabilities in active markets. Level 2 — Include other inputs that are based upon quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active, and model-based valuation techniques for which all significant inputs are observable in the market or can be derived from observable market data. Where applicable, these models project future cash flows and discount the future amounts to a present value using market-based observable inputs including interest rate curves, foreign exchange rates, and credit ratings. Level 3 — Unobservable inputs that are supported by little or no market activities, which would require the Company to develop its own assumptions. The fair value hierarchy also requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. The carrying amounts of certain financial instruments, such as cash equivalents, accounts receivable, accounts payable and accrued liabilities, approximate fair value due to their relatively short maturities, and low market interest rates, if applicable. Based on the fair value hierarchy, the Company classifies its cash equivalents within Level 1. This is because the Company values its cash equivalents using quoted market prices. The Company’s Level 1 securities consist of $320.5 million and $262.5 million in money market funds included in cash and cash equivalents at December 31, 2015 and December 31, 2014 , respectively.

Composition of Certain Balance

Composition of Certain Balance Sheet Items	12 Months Ended
	Dec. 31, 2015
Composition of Certain Balance Sheet Items [Abstract]
Composition of Certain Balance Sheet Items	Composition of Certain Balance Sheet Items Property and Equipment, net Property and equipment, net consisted of the following (in thousands): December 31, 2015 2014 Machinery and equipment $ 6,210 $ 5,481 Leasehold improvements 2,828 2,214 Purchased computer software 167 137 9,205 7,832 Less: accumulated depreciation and amortization (5,343 ) (4,711 ) Property and equipment, net $ 3,862 $ 3,121 Depreciation expense was $0.8 million , $0.7 million and $0.7 million for the years ended December 31, 2015, 2014 and 2013 , respectively. Other Current Liabilities Other current liabilities consisted of the following (in thousands): December 31, 2015 2014 Payroll and related expenses $ 3,774 $ 3,138 Professional services 325 1,169 Deferred rent 284 172 Other 520 496 Other current liabilities $ 4,903 $ 4,975

Net Loss Per Ordinary Share

Net Loss Per Ordinary Share	12 Months Ended
	Dec. 31, 2015
Earnings Per Share [Abstract]
Net Loss Per Ordinary Share	Net Loss Per Ordinary Share Basic net income (loss) per ordinary share is calculated by dividing net income (loss) by the weighted-average number of ordinary shares outstanding during the period. Shares used in diluted net income per ordinary share would include the dilutive effect of ordinary shares potentially issuable upon the exercise of stock options outstanding. However, potentially issuable ordinary shares are not used in computing diluted net loss per ordinary share as their effect would be anti-dilutive due to the loss recorded during the years ended December 31, 2015, 2014 and 2013 , and therefore diluted net loss per share is equal to basic net loss per share. Net loss per ordinary share was determined as follows (in thousands, except per share amounts): Year Ended December 31, 2015 2014 2013 Numerator: Net loss $ (80,612 ) $ (7,150 ) $ (40,996 ) Denominator: Weighted-average ordinary shares outstanding 30,326 24,672 18,615 Net loss per share: Basic and diluted net loss per share $ (2.66 ) $ (0.29 ) $ (2.20 ) The equivalent ordinary shares not included in diluted net loss per share because their effect would be anti-dilutive are as follows (in thousands): Year Ended December 31, 2015 2014 2013 Stock options to purchase ordinary shares 3,142 2,612 1,974

Commitment and Contingencies

Commitment and Contingencies	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitment and Contingencies	Commitments and Contingencies Operating Lease The Company has a noncancelable operating lease agreement for office and research and development space in the U.S. that expires in November 2020 with an estimated annual rent payment of approximately $2.1 million . The lease provides for approximately 50,400 of rentable square feet at a base rent that increases annually. In December 2014 , the Company entered into a noncancelable operating sublease agreement with a third party to sublease a portion of its leased facility. This sublease agreement has a three -year term which commenced in January 2015 (with options to extend for another year). The Company expects to receive future minimum payments from its sublease of $0.5 million and $0.3 million in 2016 and 2017, respectively, which is an offset to the lease payments below. In August 2015 , the Company entered into an agreement to lease 6,258 square feet of office space in Dublin, Ireland. This lease has a term of 10 years from commencement and provides for an option to terminate the lease at the end of the fifth year of the term. It is also subject to a rent review every five years. As a result of this noncancelable operating lease agreement, the Company is obligated to make lease payments totaling approximately €2.0 million , or $2.2 million as converted using exchange rate as of December 31, 2015 , over the term of the lease, assuming current lease payments. Of this obligation, approximately $2.1 million remain outstanding as of December 31, 2015 . Future minimum payments under noncancelable operating leases and future minimum rentals to be received under the sublease as of December 31, 2015 , are as follows (in thousands): Year Ended December 31, Operating Lease Sublease Rental 2016 $ 2,126 $ (523 ) 2017 2,242 (316 ) 2018 2,322 — 2019 2,406 — 2020 2,283 — Thereafter 1,054 — Total $ 12,433 $ (839 ) The Company recognizes rent expense on a straight-line basis over the noncancelable lease term and records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. Where leases contain escalation clauses, rent abatements, and/or concessions, such as rent holidays and landlord or tenant incentives or allowances, the Company applies them in the determination of straight-line rent expense over the lease term. The Company records the tenant improvement allowance as deferred rent and associated expenditures as leasehold improvements that are being amortized over the shorter of their estimated useful life or the term of the lease. The Company records payments received from its sublease as offset against the current period rent expense. Rent expense was $1.9 million , $1.8 million and $1.3 million for the years ended December 31, 2015, 2014 and 2013, respectively . Indemnity Obligations The Company has entered into indemnification agreements with its current, and former, directors and officers and certain key employees. These agreements contain provisions that may require the Company, among other things, to indemnify such persons against certain liabilities that may arise because of their status or service and advance their expenses incurred as a result of any indemnifiable proceedings brought against them. The obligations of the Company pursuant to the indemnification agreements continue during such time as the indemnified person serves the Company and continues thereafter until such time as a claim can be brought. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited; however, the Company has a director and officer liability insurance policy that limits its exposure and enables the Company to recover a portion of any future amounts paid. As a result of its insurance policy coverage, the Company believes the estimated fair value of these indemnification agreements is minimal. Accordingly, the Company had no liabilities recorded for these agreements as of December 31, 2015 and 2014 . Commitments In the normal course of business, the Company enters into various firm purchase commitments primarily related to research and development activities. As of December 31, 2015 , the Company had non-cancelable purchase commitments to suppliers for $7.1 million of which $3.1 million is included in accrued current liabilities, and contractual obligations under license agreements of $1.5 million of which $0.1 million is included in accrued current liabilities. The following is a summary of the Company's non-cancelable purchase commitments and contractual obligations as of December 31, 2015 (in thousands): Total 2016 2017 2018 2019 2020 Thereafter Purchase Obligations $ 7,081 $ 7,028 $ 44 $ 9 $ — $ — $ — Contractual obligations under license agreements (1) 1,507 233 123 123 123 93 812 Total $ 8,588 $ 7,261 $ 167 $ 132 $ 123 $ 93 $ 812 (1) Excludes future obligations pursuant to the cost-sharing arrangement under the Company's License Agreement with Roche. Amounts of such obligations, if any, cannot be determined at this time.

Roche License Agreements

Roche License Agreements	12 Months Ended
	Dec. 31, 2015
Collaborative Agreement [Abstract]
Roche License Agreements	Roche License Agreement Overview In December 2013 , the Company entered into the License Agreement with Roche to develop and commercialize certain antibodies that target α- synuclein, including PRX002, which are referred to collectively as “Licensed Products.” Upon the effectiveness of the License Agreement in January 2014, the Company granted to Roche an exclusive, worldwide license to develop, make, have made, use, sell, offer to sell, import and export the Licensed Products. The Company retained certain rights to conduct development of the Licensed Products and an option to co-promote PRX002 in the U.S. During the term of the License Agreement, the Company and Roche will work exclusively with each other to research and develop antibody products targeting α- synuclein potentially including incorporation of Roche’s proprietary Brain Shuttle™ technology to potentially increase delivery of therapeutic antibodies to the brain. The License Agreement provides that Roche would make an upfront payment to the Company of $30.0 million , which was received in February 2014, and the clinical milestone payment of $15.0 million triggered by the initiation of the Phase 1 study for PRX002 in the clinic, which was received in May 2014. For PRX002, Roche is also obligated to pay: • up to $380.0 million upon the achievement of development, regulatory and various first commercial sales milestones; • up to an additional $175.0 million in ex-U.S. commercial sales milestones; and • tiered, high single-digit to high double-digit royalties in the teens on ex-U.S. annual net sales, subject to certain adjustments. Roche bears 100% of the cost of conducting the research activities under the License Agreement. In the U.S., the parties will share all development and commercialization costs, as well as profits, all of which will be allocated 70% to Roche and 30% to the Company, for PRX002 in the Parkinson’s disease indication, as well as any other Licensed Products and/or indications for which the Company opts in to participate in co-development and co-funding. After the completion of specific clinical trial activities, the Company may opt out of the co-development and cost and profit sharing on any co-developed Licensed Products and instead receive U.S. commercial sales milestones totaling up to $155.0 million and tiered, single-digit to high double-digit royalties in the teens based on U.S. annual net sales, subject to certain adjustments, with respect to the applicable Licensed Product. The Company filed an Investigational New Drug Application ("IND") with the FDA for PRX002 and subsequently initiated a Phase 1 study in 2014. Following the Phase 1 study, Roche will be primarily responsible for developing, obtaining and maintaining regulatory approval for, and commercializing Licensed Products. Roche will also become responsible for the clinical and commercial manufacture and supply of Licensed Products within a defined time period following the effective date of the License Agreement. In addition, the Company has an option under the License Agreement to co-promote PRX002 in the U.S. in the Parkinson’s disease indication. If the Company exercises such option, it may also elect to co-promote additional Licensed Products in the U.S. approved for Parkinson’s disease. Outside the U.S., Roche will have responsibility for developing and commercializing the Licensed Products. Roche bears all costs that are specifically related to obtaining or maintaining regulatory approval outside the U.S. and will pay the Company a variable royalty based on annual net sales of the Licensed Products outside the U.S. While Roche will record product revenue from sales of the Licensed Products, the Company and Roche will share in the net profits and losses of sales of the PRX002 for the Parkinson's disease indication in the U.S. on a 70% / 30% basis with the Company receiving 30% of the profit and losses provided that the Company has not exercised its opt-out right. The License Agreement continues on a country-by-country basis until the expiration of all payment obligations under the License Agreement. The License Agreement may also be terminated (i) by Roche at will after the first anniversary of the effective date of the License Agreement, either in its entirety or on a Licensed Product-by-Licensed Product basis, upon 90 days’ prior written notice to the Company prior to first commercial sale and 180 days’ prior written notice to Prothena after first commercial sale, (ii) by either party, either in its entirety or on a Licensed Product-by-Licensed Product or region-by-region basis, upon written notice in connection with a material breach uncured 90 days after initial written notice, and (iii) by either party, in its entirety, upon insolvency of the other party. The License Agreement may be terminated by either party on a patent-by-patent and country-by-country basis if the other party challenges a given patent in a given country. The Company’s rights to co-develop Licensed Products under the License Agreement will terminate if the Company commences certain studies for certain types of competitive products. The Company’s rights to co-promote Licensed Products under the License Agreement will terminate if the Company commences a Phase 3 study for such competitive products. The License Agreement cannot be assigned by either party without the prior written consent of the other party, except to an affiliate of such party or in the event of a merger or acquisition of such party, subject to certain conditions. The License Agreement also includes customary provisions regarding, among other things, confidentiality, intellectual property ownership, patent prosecution, enforcement and defense, representations and warranties, indemnification, insurance, and arbitration and dispute resolution. Collaboration Accounting The License Agreement was evaluated under ASC 808, "Collaborative Agreements". At the outset of the contract, the Company concluded that this agreement does not qualify as a collaboration under ASC 808 because Prothena is not an active participant as a result of the opt-out provision. The Company believes that Roche is the principal in the sales transactions with third parties as it is the primary obligor, bearing inventory and credit risk. The Company will record its share of pre-tax commercial profit generated from the collaboration with Roche, as collaboration revenue when the profit share can be reasonably estimated and collectability is reasonably assured. Prior to commercialization of a Licensed Product, the Company's portion of the expenses related to the License Agreement reflected on its income statement will be limited to R&D expenses. After commercialization, if the Company opts-in to co-detail commercialization expenses related to commercial capabilities, including the establishment of a field sales force and other activities to support the Company’s commercialization efforts, will be recorded as SG&A expense and will be factored into the computation of the profit and loss share. The Company will record the portion of the net receivable related to commercialization activities as collaboration revenue. Multiple Element Accounting The License Agreement was evaluated under ASC 605-25, "Multiple Element Arrangements". The License Agreement includes the following deliverables: (1) an exclusive royalty bearing license, with the right to sublicense to develop and commercialize certain antibodies that target α- synuclein, including PRX002 delivered at the effective date; (2) the Company's obligation to supply clinical material as requested by Roche for a period up to twelve months; (3) the Company's obligation to provide manufacturing related services to Roche for a period up to twelve months; (4) the Company's obligation to provide development activities under the development plan including the preparation and filing of the IND and initiation of the Phase I clinical trial estimated to be carried out over the next two years; and (5) the Company's obligation to provide indeterminate research services for up to three years at rates that are not significantly discounted and fully reimbursable by Roche. All of the deliverables were deemed to have stand-alone value and to meet the criteria to be accounted for as separate units of accounting under ASC 605-25. Factors considered in the determination included, among other things, for the license, the research and development capabilities of Roche and Roche’s sublicense rights, and for the remaining deliverables the fact that they are not proprietary and can be and have been provided by other vendors. The amount of allocable arrangement consideration is limited to amounts that are fixed or determinable excluding refund rights, concessions or performance bonuses. As such, the Company will exclude from such allocable consideration the milestone payments and royalties regardless of the probability of receipt because such payments are not considered fixed or determinable. Such milestone payments and royalties will be evaluated separately as the related contingencies are resolved. Consideration for research services were not allocated as the amount is not fixed and determinable and is not at a significant incremental discount. There are no refund rights, concessions or performance bonuses to consider. The Company allocated the fixed and determinable consideration to the license and other deliverables using the relative selling price method based on the best estimate of selling price for the license and third party evidence for the remaining deliverables. The best estimate of selling price for the license was based on a discounted cash flow model. The key assumptions used in the discounted cash flow model used to determine the best estimate of selling price for the license granted to Roche under the License Agreement included the market opportunity for commercialization of PRX002 in the U.S. and the Royalty Territory (for jointly funded products the Royalty Territory is worldwide except for the U.S. for all Licensed Products that are not jointly funded the Royalty Territory is worldwide), the probability of successfully developing and commercializing PRX002, the remaining development costs for PRX002, and the estimated time to commercialization of PRX002. The Company’s discounted cash flow model included several market conditions and entity-specific inputs, including the likelihood that clinical trials will be successful, the likelihood that regulatory approval will be obtained and the product commercialized, the appropriate discount rate, the market locations, size and potential market share of the product, the expected life of the product, and the competitive environment for the product. The market assumptions were generated using a patient-based forecasting approach, with key epidemiological, market penetration, dosing, compliance, length of treatment, and pricing assumptions derived from primary and secondary market research, referenced from third-party sources. The Company concluded that a change in the assumptions used to determine the BESP of the license deliverable would not have a significant effect on the allocation of arrangement consideration. Based on the relative selling price method, the amount that the Company recognized on the effective date of the agreement concurrent with the delivery of the license and know-how was limited to the lesser of the amount otherwise allocable using the relative selling price method, which based on the discounted cash flow model was determined to be $35.6 million , or the non-contingent amount which was the $30.0 million upfront fee. As the remaining deliverables are delivered, any consideration received will be allocated to license revenue and the other deliverables based on their relative percentages until such time as the full allocated consideration of $35.6 million has been recognized as license revenue, and the balance of the monetary consideration will be recorded as an offset against R&D expenses. The Company recognized the $30.0 million upfront payment as collaboration license revenue in the first quarter of 2014. Under this agreement, the Company recognizes research reimbursement as collaboration revenue as earned. The Company recognized $1.4 million as collaboration revenue for research reimbursement from Roche for the year ended December 31, 2015 , as compared to $1.7 million for the year ended December 31, 2014 . Cost sharing payments to Roche are recorded as R&D expenses. The Company recognized $2.9 million in R&D expenses for payments made to Roche during the year ended December 31, 2015 , as compared to $1.4 million for the year ended December 31, 2014 . Reimbursement for development costs from Roche under the cost-sharing arrangement were allocated between license revenue and an offset to R&D expenses based on the relative selling price method until the full allocated consideration of $35.6 million was recognized as license revenue, after which the full reimbursement is recorded as an offset to R&D expenses. In the year ended December 31, 2015, the Company reached the full allocated consideration of $35.6 million recognized as license revenue; accordingly, future development revenue will be recorded as an offset to R&D expenses. Reimbursement for development costs from Roche during the year ended December 31, 2015 was $5.1 million , of which $0.2 million , was recognized as collaboration license revenue and $4.9 million , was recognized as an offset to R&D expenses. Reimbursement for development costs from Roche during the years ended December 31, 2014 was $6.0 million , of which $5.3 million was recognized as collaboration license revenue and $0.7 million , respectively were recognized as an offset to R&D expenses. Milestone Accounting In April 2014, the Company together with Roche initiated a Phase 1 clinical trial of PRX002. As a result of this initiation, the Company received a $15.0 million milestone payment from Roche under the License Agreement. The Company concluded that the $15.0 million clinical milestone triggered upon the initiation of the Phase 1 study from PRX002 in the clinic is consistent with the definition of a substantive milestone included in ASU No. 2010-17, "Milestone Method of Revenue Recognition". Factors considered in this determination included scientific and regulatory risk that must be overcome to achieve each milestone, the level of effort and investment required to achieve the milestone, and the monetary value attributed to the milestone. Accordingly, the Company recognized payments related to the achievement of this milestone when the milestone was achieved. The milestone payment was allocated to the units of accounting based on the relative selling price method for income statement classification purposes. In the year ended December 31, 2014 , the Company recognized $13.3 million of the $15.0 million milestone payment as collaboration revenue and $1.7 million as an offset to R&D expenses. The clinical and regulatory milestones under the License Agreement after the point at which the Company could opt-out are not considered to be substantive due to the fact that active participation in the development activities that generate the milestones is not required by the License Agreement, and the Company can opt-out of these activities. There are no refund or claw-back provisions and the milestones are uncertain of occurrence even after the Company has opted out. Based on this determination, these milestones will be recognized similar to the commercial milestone, which will be accounted for as contingent revenue payments with revenue recognized upon achievement of the milestone assuming all revenue recognition criteria are met. The Company did not achieve any of the clinical and regulatory milestones under the License Agreement during the year ended December 31, 2015 .

Shareholders' Equity

Shareholders' Equity	12 Months Ended
	Dec. 31, 2015
Equity [Abstract]
Shareholders' Equity	Shareholders' Equity Ordinary Shares As of December 31, 2015 , the Company had 100,000,000 ordinary shares authorized for issuance with a par value of $0.01 per ordinary share and 31,744,102 ordinary shares issued and outstanding. Each ordinary share is entitled to one vote and, on a pro rata basis, to dividends when declared and the remaining assets of the Company in the event of a winding up. Euro Deferred Shares As of December 31, 2015 , the Company had 10,000 Euro Deferred Shares authorized for issuance with a nominal value of €22 per share. No Euro Deferred Shares are outstanding at December 31, 2015 . The rights and restrictions attaching to the Euro Deferred Shares rank pari passu with the ordinary shares and are treated as a single class in all respects. October 2013 Offering In October 2013, the Company completed an underwritten public offering of an aggregate of 6,796,500 of its ordinary shares at a public offering price of $22.00 per share, which consisted of 4,177,079 newly issued ordinary shares sold by the Company and 2,619,421 ordinary shares sold by Janssen Pharmaceutical, a wholly-owned subsidiary of Johnson & Johnson, as the selling shareholder. The Company received aggregate net proceeds of approximately $84.5 million , after deducting the underwriting discount and estimated offering costs. The Company did not receive any proceeds from the sale of 2,619,421 ordinary shares sold, which represented Janssen Pharmaceutical's entire shareholding in Prothena. During the year ended December 31, 2013 underwriting discounts and offering costs of $7.4 million were recorded as an offset to the proceeds and recorded in additional paid in capital. February 2014 Offering In February 2014 , Elan Science One Limited ("ESOL"), an indirect wholly owned subsidiary of Perrigo, sold 3,182,253 ordinary shares of Prothena at a price to the public of $26.00 per ordinary share, before the underwriting discount. As a result, ESOL and Perrigo no longer own any ordinary shares of Prothena. The Company did not receive any of the proceeds from the offering, and the total number of the Company's ordinary shares outstanding did not change as a result of this offering. The Company paid the costs associated with the sale of these ordinary shares (other than the underwriting discount, fees and disbursements of counsel for the selling shareholder) pursuant to a Subscription and Registration Rights Agreement dated November 8, 2012 by and between the Company, Elan and ESOL. June 2014 Offering In July 2014, the Company completed an underwritten public offering of an aggregate of 5,462,500 of its ordinary shares at a public offering price of $22.50 per ordinary share. The Company received aggregate net proceeds of approximately $117.4 million (of which $102.5 million and $14.9 million was received in June 2014 and July 2014, respectively), after deducting the underwriting discount and estimated offering costs. For the year ended December 31, 2014 underwriting discount and offering expense of $5.5 million were classified as an offset to the proceeds and recorded in additional paid in capital on the balance sheet. April 2015 Offering In April 2015 , the Company completed an underwritten public offering of an aggregate of 3,795,000 of its ordinary shares at a public offering price of $37.00 per ordinary share. The Company received aggregate net proceeds of approximately $131.5 million , after deducting the underwriting discount and estimated offering costs. For the year ended December 31, 2015 underwriting discount and offering expense of $8.9 million were classified as an offset to the proceeds and recorded in additional paid in capital on the balance sheet.

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Compensation	Share-Based Compensation Amended and Restated 2012 Long Term Incentive Plan (“LTIP”) Employees and consultants of the Company, its subsidiaries and affiliates, as well as members of the Board, are eligible to receive equity awards under the LTIP. The LTIP provides for the grant of stock options, including incentive stock options and nonqualified stock options, stock appreciation rights (“SARS”), restricted shares, restricted share units ("RSUs"), cash or stock-based performance awards and other share-based awards to eligible individuals. Options under the LTIP may be granted for periods up to ten years. All options issued to date have had a ten year life. The Company granted 1,155,300 , 720,500 and 1,978,000 share options during the years ended December 31, 2015 , 2014 and 2013 , respectively, under the LTIP. The Company's option awards generally vest over four years. The aggregate number of ordinary shares authorized for issuance under the LTIP is 5,550,000 ordinary shares and as of December 31, 2015 , 1,777,295 ordinary shares remain available for grant and options to purchase 3,142,364 ordinary shares granted from the LTIP were outstanding with a weighted-average exercise price of approximately $21.36 per share. Share-based Compensation Expense The Company estimates the fair value of share-based compensation on the date of grant using an option-pricing model. The Company uses the Black-Scholes model to value share-based compensation, excluding RSUs, which the Company values using the fair market value of its ordinary shares on the date of grant. The Black-Scholes option-pricing model determines the fair value of share-based payment awards based on the share price on the date of grant and is affected by assumptions regarding a number of complex and subjective variables. These variables include, but are not limited to, the Company’s share price, volatility over the expected life of the awards and actual and projected employee stock option exercise behaviors. Since the Company does not have sufficient historical employee share option exercise data, the simplified method has been used to estimate the expected life of all options. Prior to 2015, the expected volatility was based on historical stock volatilities of several of the Company's publicly traded comparable companies over a period equal to the expected life of the options, as the Company did not have a long enough trading history to use the volatility of its own ordinary shares. Starting in 2015, the expected volatility was based on a combination of historical volatility for the Company's stock and the historical volatilities of several of the Company's publicly traded comparable companies. Although the fair value of share options granted by the Company is estimated by the Black-Scholes model, the estimated fair value may not be indicative of the fair value observed in a willing buyer and seller market transaction. As share-based compensation expense recognized in the Consolidated Financial Statements is based on awards ultimately expected to vest, it has been reduced for estimated forfeitures. Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from estimates. Forfeitures were estimated based on estimated future turnover and historical experience. Share-based compensation expense will continue to have an adverse impact on the Company’s results of operations, although it will have no impact on its overall financial position. The amount of unearned share-based compensation currently estimated to be expensed from now through the year 2018 related to unvested share-based payment awards at December 31, 2015 is $31.2 million . The weighted-average period over which the unearned share-based compensation is expected to be recognized is 2.5 years . If there are any modifications or cancellations of the underlying unvested securities, the Company may be required to accelerate, increase any remaining unearned share-based compensation expense. Future share-based compensation expense and unearned share-based compensation will increase to the extent that the Company grants additional equity awards. Share-based compensation expense recorded in these Consolidated Financial Statements for the years ended December 31, 2015 , 2014 and 2013 and was based on awards granted under the LTIP. The following table summarizes share-based compensation expense for the periods presented (in thousands): Year Ended December 31, 2015 2014 2013 Research and development (1) $ 4,301 $ 2,270 $ 980 General and administrative 6,113 3,327 2,148 Total share-based compensation expense $ 10,414 $ 5,597 $ 3,128 _________________ (1) Includes $42,000 , $108,000 and $320,000 for the years ended December 31, 2015 , 2014 and 2013 , respectively, of share-based compensation expense related to options granted to a consultant. For the years ended December 31, 2015 , 2014 and 2013 , the Company recognized tax benefits from share-based awards of $1.7 million , $1.0 million and $0.4 million , respectively. The fair value of the options granted to employees and non-employee directors during the years ended December 31, 2015 , 2014 and 2013 was estimated as of the grant date using the Black-Scholes option-pricing model assuming the weighted-average assumptions listed in the following table: Year Ended December 31, 2015 2014 2013 Expected volatility 76.3% 84.4% 84.0% Risk-free interest rate 1.7% 1.8% 1.2% Expected dividend yield —% —% —% Expected life (in years) 6.0 6.0 6.0 Weighted average grant date fair value $23.12 $19.97 $5.22 The fair value of employee stock options is being amortized on a straight-line basis over the requisite service period for each award. Each of the inputs discussed above is subjective and generally requires significant management judgment to determine. The following table summarizes the Company’s share option activity during the year ended December 31, 2015 : Options Weighted Weighted Aggregate Outstanding at December 31, 2014 2,612,080 $ 13.13 8.45 $ 25,298 Granted 1,155,300 34.58 Exercised (561,097 ) 10.06 Canceled (63,919 ) 23.27 Outstanding at December 31, 2015 3,142,364 $ 21.36 8.14 $ 146,917 Vested and expected to vest at December 31, 2015 3,053,060 $ 21.08 8.12 $ 143,578 Vested at December 31, 2015 1,169,118 $ 11.67 7.36 $ 65,983 During the years ended December 31, 2015 , 2014 and 2013 , the total intrinsic value of options exercised was $25.5 million , $1.5 million and nil , respectively, determined as of the date of exercise. The following table summarizes information about the Company's share options outstanding as of December 31, 2015 : Options Outstanding Options Exercisable Range of Exercise Prices Number of Options Weighted - Weighted Average Exercise Price Number of Options Weighted Average Exercise Price $ 6.03 $ 6.03 383,334 7.08 $ 6.03 261,459 $ 6.03 6.41 6.41 591,665 7.08 6.41 429,024 6.41 6.65 16.42 340,250 7.29 8.01 214,946 8.51 17.36 26.47 230,838 8.32 21.53 59,458 21.59 27.81 27.81 754,550 9.19 27.81 — — 29.52 29.52 14,063 8.09 29.52 1,042 29.52 29.81 29.81 437,914 8.09 29.81 186,731 29.81 33.65 55.05 316,000 9.42 43.89 16,458 37.02 67.64 67.64 72,500 9.59 67.64 — — 70.17 70.17 1,250 9.92 70.17 — — $ 6.03 $ 70.17 3,142,364 8.14 $ 21.36 1,169,118 $ 11.67

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company files its U.S. and Irish income tax returns and income taxes are presented in the Consolidated Financial Statements using the asset and liability method prescribed by the accounting guidance for income taxes. Income (loss) before provision for income taxes by country for each of the fiscal periods presented is summarized as follows (in thousands): Year Ended December 31, 2015 2014 2013 Ireland $ (83,009 ) $ (9,714 ) $ (42,523 ) U.S. 3,098 3,375 1,942 Loss before provision for income taxes $ (79,911 ) $ (6,339 ) $ (40,581 ) Components of the provision for income taxes for each of the fiscal periods presented consisted of the following (in thousands): Year Ended December 31, 2015 2014 2013 Current: U.S. Federal $ 1,663 $ 1,855 $ 958 U.S. State 1 1 (5 ) Ireland — — — Total current provision $ 1,664 $ 1,856 $ 953 Deferred: U.S. Federal $ (963 ) $ (1,045 ) $ (538 ) U.S. State — — — Ireland — — — Total deferred benefit (963 ) (1,045 ) $ (538 ) Total provision for income taxes $ 701 $ 811 $ 415 Excess tax benefits associated with share-based compensation deductions are credited to shareholders' equity. The reductions in income taxes payable resulting from share-based compensation deductions that were credited to stockholder's equity were approximately $3.9 million , $0.2 million and nil for the years ended December 31, 2015 , 2014 and 2013 , respectively. The provision for income taxes differs from the statutory tax rate of 12.5% applicable to Ireland primarily due to Irish net operating losses for which a tax provision benefit is not recognized and due to U.S. income taxed at different rates. Following is a reconciliation between income taxes computed at the Irish statutory tax rate and the provision for income taxes for each of the fiscal periods presented (in thousands): Year Ended December 31, 2015 2014 2013 Taxes at the Irish statutory tax rate of 12.5% $ (9,989 ) $ (792 ) $ (5,073 ) Income tax at rates other than applicable statutory rate 446 705 (3,169 ) Change in valuation allowance 12,594 2,444 10,365 Share-based payments 214 74 164 Tax credits (2,712 ) (1,643 ) (1,921 ) Other 148 23 49 Provision for income taxes $ 701 $ 811 $ 415 Deferred income taxes reflect the net tax effect of temporary differences between the carrying amount of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of the Company’s net deferred tax assets as of December 31, 2015 and 2014 are as follows (in thousands): December 31, 2015 2014 Deferred tax assets: Net operating losses $ 26,676 $ 16,322 Tax credits 2,473 1,139 Accruals 1,117 1,016 Share-based compensation 3,769 2,225 Gross deferred tax assets 34,035 20,702 Valuation allowance (31,185 ) (18,815 ) Net deferred tax assets 2,850 1,887 Deferred tax liability — — Net deferred tax assets $ 2,850 $ 1,887 The Company's deferred tax assets are composed primarily of its Irish subsidiaries' net operating loss carryovers, state net operating loss carryforwards available to reduce future taxable income of the Company's U.S. subsidiary, federal and state tax credit carryforwards and other temporary differences. The Company maintains a valuation allowance against certain U.S. federal and state and Irish deferred tax assets. Each reporting period, the Company evaluates the need for a valuation allowance on its deferred tax assets by jurisdiction. Recognition of deferred tax assets is appropriate when realization of such assets is more likely than not. Based upon the weight of available evidence, especially the uncertainties surrounding the realization of deferred tax assets through future taxable income, the Company believes it is not yet more likely than not that the deferred tax assets will be fully realizable. Accordingly, the Company has provided a valuation allowance of $31.2 million against its deferred tax assets as of December 31, 2015 in relation to deferred tax assets arising from tax credits and net operating losses. The deferred tax assets recognized net of the valuation allowance, $2.9 million as of December 31, 2015, consist of U.S. federal temporary differences. Due to consistent U.S. operating income, the Company expects to realize such deferred tax assets. The net increase of $12.4 million in the valuation allowance during the year ended December 31, 2015 was primarily due to net operating losses of the Company's Irish subsidiaries, and to a lesser extent from share-based compensation, US federal and state tax credits. As of December 31, 2015 , certain of the Company's Irish subsidiaries had trading loss carryovers of $179.7 million and non-trading loss carryovers of $8.0 million , each of which can be carried forward indefinitely but are limited to the same trade/trades. In addition, as of December 31, 2015 , the Company had federal and state net operating loss carryforwards of approximately $0.7 million and $36.4 million , respectively, which are available to reduce future taxable income for the Company's U.S. subsidiary, if any. If not utilized, the federal net operating loss carryforward begins to expire in 2035 and state net operating loss carryforward begins expiring in 2032 . The Company also has federal and California research and development credit carryforwards of $3.9 million and $2.5 million , respectively, at December 31, 2015 . The federal research and development credit carryforwards will expire starting in 2033 if not utilized. The California tax credits can be carried forward indefinitely. As a result of certain realization requirements of ASC Topic 718, "Compensation - Stock Compensation" the table of deferred tax assets does not include certain deferred tax assets as of December 31, 2015 that arose directly from tax deductions related to equity compensation that are greater than the compensation recognized for financial reporting. Equity will be increased by $3.5 million if and when such deferred tax assets are ultimately realized. We use ASC Topic 740, "Income Taxes" ordering when determining when excess tax benefits have been realized. Cumulative unremitted earnings of the Company’s U.S. subsidiary total approximately $6.5 million at December 31, 2015 The Company's U.S. subsidiary's cash balance at December 31, 2015 is committed for its working capital needs. No taxes have been provided for the unremitted earnings as any tax basis differences relating to investments in this overseas subsidiary are considered to be permanent in duration. Unremitted earnings may be subject to U.S. withholding taxes (potentially at 5% ) and Irish taxes (potentially at a rate of 12.5% ) if they were to be distributed as dividends. However, Ireland allows a credit against Irish taxes for U.S. tax withheld and as of December 31, 2015 the Company's net operating losses in Ireland are sufficient to offset any potential dividend income received from its U.S. subsidiary. A reconciliation of the beginning and ending amounts of unrecognized tax benefits is as follows (in thousands): 2015 2014 Gross Unrecognized Tax Benefits at January 1 $ 383 $ 480 Additions for tax positions taken in the current year 678 253 Reductions for tax positions taken in the prior year — (350 ) Gross Unrecognized Tax Benefits at December 31 $ 1,061 $ 383 If recognized, none of our unrecognized tax benefits as of December 31, 2015 would reduce our annual effective tax rate, primarily due to corresponding adjustments to our deferred tax valuation allowance. As of December 31, 2015 , we have not recorded a liability for potential interest or penalties. We also do not expect our unrecognized tax benefits to change significantly over the next 12 months. The tax years 2012 to 2015 remain subject to examination by the U.S taxing authorities and the tax years 2011 to 2015 remain subject to examination by the Irish taxing authorities.

Employee Retirement Plan

Employee Retirement Plan	12 Months Ended
	Dec. 31, 2015
Compensation and Retirement Disclosure [Abstract]
Employee Retirement Plan	Employee Retirement Plan In the United States, the Company provides a qualified retirement plan under section 401(k) of the Internal Revenue Code (“IRC”) under which participants may contribute up to 100% of their eligible compensation, subject to maximum deferral limits specified by the IRC. In addition, the Company contributes 3% of each participating employee’s eligible compensation, subject to limits specified by the IRC, on a quarterly basis. Further, the Company may make an annual discretionary matching and/or profit sharing contribution as determined solely by the Company. The Company recorded total expense for matching contributions of $0.4 million , $0.3 million and $0.3 million for the years ended December 31, 2015 , 2014 and 2013 , respectively. In Ireland, the Company operates a defined contribution plan in which it contributes up to 7.5% of an employee’s eligible earnings. The Company recorded expense of $51,000 and $19,000 in the years ended December 31, 2015 and 2014 , respectively, and none in 2013 in connection with the contributions we made under the Irish defined contribution plan.

Related Parties

Related Parties	12 Months Ended
	Dec. 31, 2015
Related Party Transactions [Abstract]
Related Parties	Related Parties Prior to December 21, 2012, the Company operated as part of Elan and not as a separate stand-alone entity. Effective December 20, 2012, the Prothena Business separated from Elan. In connection with the separation, a wholly owned subsidiary of Elan acquired an 18% interest in the Company (as calculated immediately following the separation). Elan was subsequently acquired by Perrigo, in December 2013, and such 3,182,253 ordinary shares were held by ESOL, an indirect wholly owned subsidiary of Perrigo, as of December 31, 2013. February 2014 Offering In February 2014 , ESOL sold 3,182,253 ordinary shares of Prothena at a price to the public of $26.00 per ordinary share, before the underwriting discount. As a result, ESOL and Perrigo no longer owned any ordinary shares of Prothena as of such sale. The Company did not receive any of the proceeds from the offering, and the total number of the Company's ordinary shares outstanding did not change as a result of this offering. The Company paid the expenses associated with the sale of these ordinary shares (other than the underwriting discount, fees and disbursements of counsel for the selling shareholder) pursuant to a Subscription and Registration Rights Agreement dated November 8, 2012 by and among the Company, Elan and ESOL. Agreements with Elan The related party revenue recognized by the Company for the years ended December 31, 2014 and 2013 consisted of fees arising from R&D services provided to Elan. There was no related party revenue recognized by the Company for the year ended December 31, 2015 . The Company entered into certain agreements with Elan, including the Transitional Services Agreement and the R&D Services Agreement. Transitional Services Agreement In December 2012, as amended in March 2013, the Company entered into a Transitional Services Agreement (“TSA”) with Elan under which Elan provided to the Company, and the Company provided to Elan, specified services to help ensure an orderly transition following the Separation and Distribution. The services provided by Elan under the Transitional Services Agreement included chemistry, manufacturing and controls/quality assurance, information technology services, facilities services, company secretarial services, finance services, legal services, compliance services and human resources services. The payment terms of the agreement generally provided that the Company would pay Elan for the time spent by each Elan employee providing the services, which will be calculated by the portion of the employee’s time dedicated to the provision of the services, plus 40% . Similarly, Elan would pay the Company for the time spent by each of the Company’s employee providing services to Elan, which would be an agreed percentage of the employee’s time, based on the cost of providing those services plus 40% and including, as applicable, any fees for any services from Elan or the Company provided by third party providers and invoiced to the recipient at cost. TSA expenses recognized during the years ended December 31, 2015 , 2014 and 2013 respectively, were $nil , $nil and $0.5 million (of which $0.1 million was included in R&D expenses and $0.4 million was included in G&A expenses), respectively. The TSA expired on December 31, 2013. R&D Services Agreement In December 2012, as amended in March 2013, the Company entered into a Research and Development Services Agreement (“RDSA”) with Elan pursuant to which the Company provided certain R&D services to Elan. Either party was entitled to terminate the RDSA at any time by notice in writing to the other party if there has been an uncured material breach by the other party or if the other party becomes insolvent or if the other party is in breach of any of its confidentiality obligations under the agreement. This RDSA expired in December 2014 at the end of its two -year term. The services provided for under the RDSA included support for the ELND005 program (which include the provision of expert advice and opinion in the areas of nonclinical safety/toxicology and pharmacology, regulatory support for nonclinical sections of pertinent documents, conducting and interpreting externally conducted nonclinical studies, and support in respect of the identification and maintenance of nonclinical expert advisors as required). These services were substantially similar to research services performed by the Company for Elan prior to the separation and distribution. The payment terms of the RDSA provided that Elan would pay the Company: (i) a fixed charge of $500,000 per year based on a charge for two of the Company’s employees providing the services at a rate of $250,000 each per annum, (ii) if the $500,000 fixed charge has been paid in any year, a variable charge of $250,000 per year for any additional employee that provides services for such year (calculated pro rata based on the number of days the employee provides services in such year), (iii) research costs including direct overheads, and (iv) a mark-up of 10% applied to the fixed charge, variable charge (if any) and research costs such that the total payment reflects a cost-plus standard. Revenue recognized by the Company for the years ended December 31, 2014 and 2013 included fees arising from R&D services provided to Elan of $534,000 and $676,000 , respectively. Since the RDSA with Elan terminated in December 2014, the Company did not have any related-party revenue for the year ended December 31, 2015 .

Quarterly Financial Information

Quarterly Financial Information (Unaudited)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Information (Unaudited)	Selected Quarterly Financial Information (Unaudited) The following table presents selected unaudited consolidated financial data for each of the eight quarters in the two-year period ended December 31, 2015 . In the Company's opinion, this unaudited information has been prepared on the same basis as the audited information and includes all adjustments (consisting of only normal recurring adjustments) necessary for a fair statement of the financial information for the period presented. Net loss per share - basic and diluted, for the four quarters of each fiscal year may not sum to the total for the fiscal year because of the different number of shares outstanding during each period (in thousands, except per share data): Quarter First Second Third Fourth Year Ended December 31, 2015 Revenues $ 593 $ 278 $ 429 $ 307 Operating expenses $ 15,622 $ 18,313 $ 23,090 $ 24,519 Net loss $ (15,202 ) $ (18,277 ) $ (22,976 ) $ (24,157 ) Net loss per share - basic $ (0.55 ) $ (0.59 ) $ (0.73 ) $ (0.76 ) Net loss per share - diluted $ (0.55 ) $ (0.59 ) $ (0.73 ) $ (0.76 ) Year Ended December 31, 2014 Revenues $ 32,234 $ 15,121 $ 1,486 $ 2,013 Operating expenses $ 14,215 $ 13,552 $ 14,618 $ 15,118 Net income (loss) $ 17,852 $ 1,290 $ (13,182 ) $ (13,110 ) Net income (loss) per share - basic $ 0.82 $ 0.06 $ (0.48 ) (0.48 ) Net income (loss) per share - diluted $ 0.78 $ 0.06 $ (0.48 ) $ (0.48 )

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2015
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events January 2016 Offering In January 2016 , the Company completed an underwritten public offering of an aggregate of 2,587,500 of its ordinary shares at a public offering price of $53.00 per ordinary share. The Company received aggregate net proceeds of approximately $128.6 million , after deducting the underwriting discount and estimated offering costs.

Summary of Significant Accoun21

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Basis of Preparation and Presentation of Financial Information	Basis of Preparation and Presentation of Financial Information These Consolidated Financial Statements have been prepared in accordance with the accounting principles generally accepted in the U.S. (“GAAP”) and with the instructions for Form 10-K and Regulations S-X statements. The Consolidated Financial Statements of Prothena Corporation plc are presented in U.S. dollars, which is the functional currency of the Company. These Consolidated Financial Statements include the accounts of the Company and its consolidated subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Certain amounts in the Consolidated Financial Statements have been reclassified to conform to the current year presentation.
Use of Estimates	Use of Estimates The preparation of the Consolidated Financial Statements in conformity with GAAP requires management to make judgments, estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and related disclosures. On an ongoing basis, management evaluates its estimates, including critical accounting policies or estimates related to revenue recognition, share-based compensation and research and development expenses. The Company bases its estimates on historical experience and on various other market specific and other relevant assumptions that management believes to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Because of the uncertainties inherent in such estimates, actual results may differ materially from these estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments held at financial institutions, such as commercial paper, money market funds, and other money market securities with original maturities of three months or less at date of purchase to be cash equivalents.
Property and Equipment, net	Property and Equipment, net Property and equipment, net are stated at cost less accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the related assets. Maintenance and repairs are charged to expense as incurred, and improvements and betterments are capitalized. When assets are retired or otherwise disposed of, the cost and accumulated depreciation are removed from the balance sheet and any resulting gain or loss is reflected in operations in the period realized. Depreciation and amortization periods for the Company’s property, plant and equipment are as follows: Useful Life Machinery and equipment 4-7 years Leasehold improvements Shorter of expected useful life or lease term Purchased computer software 4 years
Impairment of Long-lived Assets	Impairment of Long-lived Assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable or the estimated useful life is no longer appropriate. If circumstances require that a long-lived asset be tested for possible impairment, the Company compares the undiscounted cash flows expected to be generated by the asset to the carrying amount of the asset. If the carrying amount of the long-lived asset is not recoverable on an undiscounted cash flow basis, an impairment is recognized to the extent that the carrying amount exceeds its fair value. The Company determines fair value using the income approach based on the present value of expected future cash flows. The Company’s cash flow assumptions consider historical and forecasted revenue and operating costs and other relevant factors. There were no impairment charges recorded during the years ended December 31, 2015, 2014 and 2013 .
Deferred Rent	Deferred Rent Deferred rent consists of the difference between cash payments and the recognition of rent expense on a straight-line basis for the buildings the Company occupies. The leases provide for fixed increases in minimum annual rental payments, as well as rent free periods. The total amount of rental payments due over the lease terms are being charged to rent expense ratably over the life of the leases. Tenant improvement allowances are recorded as a deferred rent liability and are amortized over the term of the lease as a reduction to rent expense.
Revenue	Revenue Recognition Revenue is recognized when earned and non-refundable, when payment is reasonably assured, and when there is no future obligation with respect to the revenue, in accordance with the terms prescribed in the applicable contract. Multiple Element Arrangements The Company's revenues are generated primarily through its license, development and commercialization agreement. These types of agreements generally contain multiple elements, or deliverables, which may include (i) licenses to the Company's technology, (ii) R&D activities to be performed on behalf of the collaborative partner, and (iii) in certain cases, services or obligations in connection with the manufacturing or supply of preclinical and clinical material. Payments to the Company under these arrangements typically include one or more of the following: non-refundable, upfront license fees; funding of research and/or development efforts; milestone payments; and royalties on future product sales. Revenue under license, development and commercialization agreements is recognized based on the performance requirements of the contract. Determinations of whether persuasive evidence of an arrangement exists and whether delivery has occurred or services have been rendered are based on management’s judgments regarding the fixed nature of the fees charged for deliverables and the collectability of those fees. The Company recognizes revenue related to license, development and commercialization agreements in accordance with the provisions of Financial Accounting Standards Board, or FASB, Accounting Standards Codification, or ASC, Topic 605-25 , “Revenue Recognition - Multiple-Element Arrangements.” The Company evaluates all deliverables within an arrangement to determine whether or not they provide value on a stand-alone basis. Based on this evaluation, the deliverables are separated into units of accounting. The arrangement consideration that is fixed or determinable at the inception of the arrangement is allocated to the separate units of accounting based on their relative selling prices. To determine the selling price of a separate deliverable, the Company uses the hierarchy as prescribed in ASC Topic 605-25 based on vendor-specific objective evidence (VSOE), third-party evidence (TPE) or best estimate of selling price (BESP). VSOE is based on the price charged when the element is sold separately and is the price actually charged for that deliverable. TPE is determined based on third party evidence for a similar deliverable when sold separately and BESP is the estimated selling price at which we would transact a sale if the elements of collaboration and license arrangements were sold on a stand-alone basis to the buyer. Payments or full reimbursements resulting from our R&D efforts for those arrangements where such efforts are considered as deliverables 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. However, such funding is recognized as a reduction of R&D expense when the Company engages in a R&D project jointly with another entity, with both entities participating in project activities and sharing costs and potential benefits of the project. Milestone Revenue The Company accounts for milestones under ASU No. 2010-17 , Milestone Method of Revenue Recognition. Under the milestone method, contingent consideration received from the achievement of a substantive milestone is recognized in its entirety in the period in which the milestone is achieved. A milestone is defined as an event (i) that can only be achieved based in whole or in part on either the entity’s performance or on the occurrence of a specific outcome resulting from the entity’s performance, (ii) for which there is substantive uncertainty at the date the arrangement is entered into that the event will be achieved, and (iii) that would result in additional payments being due to the entity. At the inception of an agreement that includes milestone payments, the Company 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. The Company evaluates factors such as the scientific, regulatory, commercial and other risks that must be overcome to achieve a particular milestone, the level of effort and investment required to achieve such milestone and whether the milestone consideration is reasonable relative to all deliverables and payment terms in the arrangement in making this assessment. The Company generally classifies each of its milestones into one of three categories: (i) clinical milestones, (ii) regulatory and development milestones, and (iii) commercial milestones. Clinical milestones are typically achieved when a product candidate advances into or completes a defined phase of clinical research. For example, a milestone payment may be due to the Company upon the initiation of a clinical trial for a new indication. Regulatory and development milestones are typically achieved upon acceptance of the submission for marketing approval of a product candidate or upon approval to market the product candidate by the FDA or other regulatory authorities. For example, a milestone payment may be due to the Company upon filing of a Biologics License Application (BLA) with the FDA. Commercial milestones are typically achieved when an approved pharmaceutical product reaches certain defined levels of net royalty sales by the licensee of a specified amount within a specified period. Commercial milestone payments and milestone payments that are not deemed to be substantive will be accounted for as a contingent revenue payment with revenue recognized when all contingencies are lifted, which is expected to be upon achievement of the milestone, assuming all revenue recognition criteria are met. Profit Share Revenue For agreements, with profit sharing arrangements, the Company will record its share of the pre-tax commercial profit as collaboration revenue when the profit sharing can be reasonably estimated and collectability is reasonably assured. Royalty Revenue The Company will recognize revenue from royalties based on licensees' sales of the Company's products or products using the Company's technologies. Royalties are recognized as earned in accordance with the contract terms when royalties from licensees can be reasonably estimated and collectability is reasonably assured.
Research and Development	Research and Development Research and development costs are expensed as incurred and include, but are not limited to, salary and benefits, share-based compensation, clinical trial activities, drug development and manufacturing prior to FDA approval and third-party service fees, including clinical research organizations and investigative sites. Costs for certain development activities, such as clinical trials, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, or information provided to the Company by its vendors on their actual costs incurred. The objective of the Company's accrual policy is to match the recording of the expenses in its Consolidated Financial Statements to the actual services received and efforts expended. As such, expense accruals related to clinical trials are recognized based on its estimate of the degree of completion of the events specified in the specific clinical study or trial contract. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the Consolidated Financial Statements as prepaid or accrued research and development. Amounts due may be fixed fee, fee for service, and may include upfront payments, monthly payments, and payments upon the completion of milestones or receipt of deliverables.
Acquired In-Process Research and Development Expense	Acquired In-Process Research and Development Expense The Company has acquired and may continue to acquire the rights to develop and commercialize new drug candidates from third parties. The upfront payments to acquire license, product or rights, as well as any future milestone payments, are immediately expensed as research and development provided that the drug has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use.
Share-based Compensation	Share-based Compensation To determine the fair value of share-based payment awards, the Company uses the Black-Scholes option-pricing model. The determination of fair value using the Black-Scholes option-pricing model is affected by the Company’s share price as well as assumptions regarding a number of complex and subjective variables. Share-based compensation expense is recognized on a straight-line basis over the requisite service period for each award. Further, share-based compensation expense recognized in the Consolidated Statements of Operations is based on awards expected to vest and therefore the amount of expense has been reduced for estimated forfeitures. If actual forfeitures differ from estimates at the time of grant they will be revised in subsequent periods. The Company bases its assumptions on historical data when available or when not available, on a peer group of companies. If factors change and different assumptions are employed in determining the fair value of share-based awards, the share-based compensation expense recorded in future periods may differ significantly from what was recorded in the current period (see Note 9 for further information).
Income Taxes	Income Taxes The Company files its own U.S. and foreign income tax returns and income taxes are presented in the Consolidated Financial Statements using the asset and liability method prescribed by the accounting guidance for income taxes. Deferred tax assets (“DTAs”) and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using the enacted tax rates projected to be in effect for the year in which the differences are expected to reverse. Net deferred tax assets are recorded to the extent the Company believes that these assets will more likely than not be realized. In making such determination, all available positive and negative evidence is considered, including scheduled reversals of deferred tax liabilities, projected future taxable income, tax planning strategies and recent financial operations. Estimates are required in determining the Company’s provision for income taxes. Some of these estimates are based on management’s interpretations of jurisdiction-specific tax laws or regulations. Various internal and external factors may have favorable or unfavorable effects on the future effective income tax rate of the business. These factors include, but are not limited to, changes in tax laws, regulations and/or rates, changing interpretations of existing tax laws or regulations, changes in estimates of prior years’ items, past and future levels of R&D spending and changes in overall levels of income before taxes. The tax benefit from an uncertain tax position is recognized only if it is more likely than not the tax position will be sustained on examination by the taxing authorities, based on the technical merits of the position. The tax benefits recognized in the financial statements from such positions are then measured based on the largest benefit that has a greater than 50% likelihood of being realized upon settlement. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs. Interest and penalties related to unrecognized tax benefits are accounted for in income tax expense.
Earnings Per Share	Net Income (loss) per Ordinary Share Basic net income (loss) per ordinary share is computed by dividing net income (loss) attributable to ordinary shareholders by the weighted average number of ordinary shares outstanding during the period. Diluted net income per ordinary share is computed by giving effect to all dilutive potential ordinary shares including options. However, potentially issuable ordinary shares are not used in computing diluted net loss per ordinary share as their effect would be anti-dilutive due to the loss recorded. In this case, diluted net loss per share is equal to basic net loss per share.
Comprehensive Loss	Comprehensive Loss Comprehensive income (loss) is comprised of net income (loss) and other comprehensive income (loss). The Company has no components of other comprehensive income (loss). Therefore net income (loss) equals comprehensive income (loss) for all periods presented and, accordingly, the Consolidated Statements of Comprehensive Income (Loss) is not presented in a separate statement.
Segment and Concentration of Risks	Segment and Concentration of Risks The Company operates in one segment. The Company’s chief operating decision maker (the “CODM”), its Chief Executive Officer, manages the Company’s operations on a consolidated basis for purposes of allocating resources. When evaluating the Company’s financial performance, the CODM reviews all financial information on a consolidated basis. Financial instruments that potentially subject the Company to concentration of credit risk consist of cash and cash equivalents and accounts receivable. The Company places its cash equivalents with high credit quality financial institutions and by policy, limits the amount of credit exposure with any one financial institution. Deposits held with banks may exceed the amount of insurance provided on such deposits. The Company has not experienced any losses on its deposits of cash and cash equivalents and its credit risk exposure is up to the extent recorded on the Company's consolidated balance sheet. Receivable from Roche as of December 31, 2015 and 2014 are amounts due from Roche entities located in the U.S. and Switzerland under the License Agreement that became effective January 22, 2014. The Company’s receivable from related party as of December 31, 2014 is from Elan located in Ireland. Revenue recorded in the Statements of Operations consists of collaboration revenue related to the upfront payment from Roche under the License Agreement, payment from Roche upon achievement of a clinical milestone and reimbursement for research and development services and fees earned from the provision of nonclinical research support to Elan, primarily in the areas of safety, toxicology and regulatory. Credit risk exposure is up to the extent recorded on the Company's Consolidated Balance Sheet. As of December 31, 2015, $3.1 million of the Company's long-lived assets were held in the U.S. and $0.8 million were in Ireland. As of December 31, 2014, all of the Company's long-lived assets were held in the U.S. The Company does not own or operate facilities for the manufacture, storage, testing or distribution of preclinical or clinical supplies of any of its drug candidates. The Company instead contracted with and relies on third-parties to manufacture, store, test and distribute all preclinical development and clinical supplies of its drug candidates, and the Company plans to continue to do so for the foreseeable future. Currently, the Company has a single source of preclinical or clinical supplies for each of its drug candidates. A delay or inability to obtain such supply could have an adverse effect on the Company's business, financial condition and results of operations.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the FASB issued Accounting Standards Update 2014-09 (ASU 2014-09), Revenue from Contracts with Customers. ASU 2014-09 supersedes the revenue recognition requirements in Revenue Recognition (Topic 605), and requires entities to recognize revenue in a way that depicts the transfer of promised goods and services to customers in an amount that reflects the consideration to which the entity expects to be entitled to in exchange for those goods or services. ASU 2014-09 is effective for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period, which for the Company is January 1, 2018. Early adoption is permitted after January 1, 2017. The standard permits the use of either retrospective or cumulative effect transition method. The Company is currently evaluating the potential impact the adoption of ASU 2014-09 will have on its consolidated financial statements. The Company has not yet selected a transition method nor has it determined the effect of the standard on its ongoing financial reporting. In November 2015, the FASB issued an Accounting Standards Update 2015-17 (ASU 2015-17), Income Taxes (Topic 740): Balance Sheet Classification of Deferred Taxes. ASU 2015-17 requires entities with classified balance sheet to present all deferred tax assets and liabilities as noncurrent and is effective for annual and interim periods beginning after December 15, 2016. However, early adoption is permitted for any financial statements that have not yet been issued. ASU 2015-17 allows entities to choose either prospective or retrospective transition, the latter of which was chosen by the Company. In these Consolidated Financial Statements the Company has presented its net deferred tax assets as noncurrent as of December 31, 2015 and 2014. The early adoption of this standard did not have a material impact on the Company's Consolidated Financial Statements since the portion of current Deferred Tax Assets in prior year of $167,000 which was reclassified as noncurrent is not significant.

Summary of Significant Accoun22

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Schedule of Property and Equipment	Depreciation and amortization periods for the Company’s property, plant and equipment are as follows: Useful Life Machinery and equipment 4-7 years Leasehold improvements Shorter of expected useful life or lease term Purchased computer software 4 years Property and equipment, net consisted of the following (in thousands): December 31, 2015 2014 Machinery and equipment $ 6,210 $ 5,481 Leasehold improvements 2,828 2,214 Purchased computer software 167 137 9,205 7,832 Less: accumulated depreciation and amortization (5,343 ) (4,711 ) Property and equipment, net $ 3,862 $ 3,121

Composition of Certain Balanc23

Composition of Certain Balance Sheet Items (Tables)	12 Months Ended
	Dec. 31, 2015
Composition of Certain Balance Sheet Items [Abstract]
Schedule of Property and Equipment	Depreciation and amortization periods for the Company’s property, plant and equipment are as follows: Useful Life Machinery and equipment 4-7 years Leasehold improvements Shorter of expected useful life or lease term Purchased computer software 4 years Property and equipment, net consisted of the following (in thousands): December 31, 2015 2014 Machinery and equipment $ 6,210 $ 5,481 Leasehold improvements 2,828 2,214 Purchased computer software 167 137 9,205 7,832 Less: accumulated depreciation and amortization (5,343 ) (4,711 ) Property and equipment, net $ 3,862 $ 3,121
Schedule of Other Current Liabilities	Other current liabilities consisted of the following (in thousands): December 31, 2015 2014 Payroll and related expenses $ 3,774 $ 3,138 Professional services 325 1,169 Deferred rent 284 172 Other 520 496 Other current liabilities $ 4,903 $ 4,975

Net Loss Per Ordinary Share (Ta

Net Loss Per Ordinary Share (Tables)	12 Months Ended
	Dec. 31, 2015
Earnings Per Share [Abstract]
Calculation of Basic and Diluted Net Income or Loss Per Ordinary Share	Net loss per ordinary share was determined as follows (in thousands, except per share amounts): Year Ended December 31, 2015 2014 2013 Numerator: Net loss $ (80,612 ) $ (7,150 ) $ (40,996 ) Denominator: Weighted-average ordinary shares outstanding 30,326 24,672 18,615 Net loss per share: Basic and diluted net loss per share $ (2.66 ) $ (0.29 ) $ (2.20 )
Ordinary Shares Equivalent Not Included in Diluted Net Loss Per Share	The equivalent ordinary shares not included in diluted net loss per share because their effect would be anti-dilutive are as follows (in thousands): Year Ended December 31, 2015 2014 2013 Stock options to purchase ordinary shares 3,142 2,612 1,974

Commitment and Contingencies (T

Commitment and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Schedule of Operating Lease Commitments	Future minimum payments under noncancelable operating leases and future minimum rentals to be received under the sublease as of December 31, 2015 , are as follows (in thousands): Year Ended December 31, Operating Lease Sublease Rental 2016 $ 2,126 $ (523 ) 2017 2,242 (316 ) 2018 2,322 — 2019 2,406 — 2020 2,283 — Thereafter 1,054 — Total $ 12,433 $ (839 )
Schedule of Contractual Obligations by Fiscal Year Maturity	The following is a summary of the Company's non-cancelable purchase commitments and contractual obligations as of December 31, 2015 (in thousands): Total 2016 2017 2018 2019 2020 Thereafter Purchase Obligations $ 7,081 $ 7,028 $ 44 $ 9 $ — $ — $ — Contractual obligations under license agreements (1) 1,507 233 123 123 123 93 812 Total $ 8,588 $ 7,261 $ 167 $ 132 $ 123 $ 93 $ 812 (1) Excludes future obligations pursuant to the cost-sharing arrangement under the Company's License Agreement with Roche. Amounts of such obligations, if any, cannot be determined at this time.

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2015
Summary of Share-Based Compensation Expense	Year Ended December 31, 2015 2014 2013 Research and development (1) $ 4,301 $ 2,270 $ 980 General and administrative 6,113 3,327 2,148 Total share-based compensation expense $ 10,414 $ 5,597 $ 3,128 _________________ (1) Includes $42,000 , $108,000 and $320,000 for the years ended December 31, 2015 , 2014 and 2013 , respectively, of share-based compensation expense related to options granted to a consultant.
Amended and Restated 2012 Long Term Incentive Plan [Member]
Fair Value of Options Granted	The fair value of the options granted to employees and non-employee directors during the years ended December 31, 2015 , 2014 and 2013 was estimated as of the grant date using the Black-Scholes option-pricing model assuming the weighted-average assumptions listed in the following table: Year Ended December 31, 2015 2014 2013 Expected volatility 76.3% 84.4% 84.0% Risk-free interest rate 1.7% 1.8% 1.2% Expected dividend yield —% —% —% Expected life (in years) 6.0 6.0 6.0 Weighted average grant date fair value $23.12 $19.97 $5.22
Summary of Company's Share Option Activity	The following table summarizes the Company’s share option activity during the year ended December 31, 2015 : Options Weighted Weighted Aggregate Outstanding at December 31, 2014 2,612,080 $ 13.13 8.45 $ 25,298 Granted 1,155,300 34.58 Exercised (561,097 ) 10.06 Canceled (63,919 ) 23.27 Outstanding at December 31, 2015 3,142,364 $ 21.36 8.14 $ 146,917 Vested and expected to vest at December 31, 2015 3,053,060 $ 21.08 8.12 $ 143,578 Vested at December 31, 2015 1,169,118 $ 11.67 7.36 $ 65,983
Summary of stock activity by exercise price	The following table summarizes information about the Company's share options outstanding as of December 31, 2015 : Options Outstanding Options Exercisable Range of Exercise Prices Number of Options Weighted - Weighted Average Exercise Price Number of Options Weighted Average Exercise Price $ 6.03 $ 6.03 383,334 7.08 $ 6.03 261,459 $ 6.03 6.41 6.41 591,665 7.08 6.41 429,024 6.41 6.65 16.42 340,250 7.29 8.01 214,946 8.51 17.36 26.47 230,838 8.32 21.53 59,458 21.59 27.81 27.81 754,550 9.19 27.81 — — 29.52 29.52 14,063 8.09 29.52 1,042 29.52 29.81 29.81 437,914 8.09 29.81 186,731 29.81 33.65 55.05 316,000 9.42 43.89 16,458 37.02 67.64 67.64 72,500 9.59 67.64 — — 70.17 70.17 1,250 9.92 70.17 — — $ 6.03 $ 70.17 3,142,364 8.14 $ 21.36 1,169,118 $ 11.67

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Loss Before Provision for Income Taxes by Region	Income (loss) before provision for income taxes by country for each of the fiscal periods presented is summarized as follows (in thousands): Year Ended December 31, 2015 2014 2013 Ireland $ (83,009 ) $ (9,714 ) $ (42,523 ) U.S. 3,098 3,375 1,942 Loss before provision for income taxes $ (79,911 ) $ (6,339 ) $ (40,581 )
Components of the Provision for Income Taxes	Components of the provision for income taxes for each of the fiscal periods presented consisted of the following (in thousands): Year Ended December 31, 2015 2014 2013 Current: U.S. Federal $ 1,663 $ 1,855 $ 958 U.S. State 1 1 (5 ) Ireland — — — Total current provision $ 1,664 $ 1,856 $ 953 Deferred: U.S. Federal $ (963 ) $ (1,045 ) $ (538 ) U.S. State — — — Ireland — — — Total deferred benefit (963 ) (1,045 ) $ (538 ) Total provision for income taxes $ 701 $ 811 $ 415
Reconciliation Between Income Taxes Computed at the Standard Irish Statutory Tax Rate and the Provision for income Taxes	Following is a reconciliation between income taxes computed at the Irish statutory tax rate and the provision for income taxes for each of the fiscal periods presented (in thousands): Year Ended December 31, 2015 2014 2013 Taxes at the Irish statutory tax rate of 12.5% $ (9,989 ) $ (792 ) $ (5,073 ) Income tax at rates other than applicable statutory rate 446 705 (3,169 ) Change in valuation allowance 12,594 2,444 10,365 Share-based payments 214 74 164 Tax credits (2,712 ) (1,643 ) (1,921 ) Other 148 23 49 Provision for income taxes $ 701 $ 811 $ 415
Significant Components of Net Deferred Tax Assets	Significant components of the Company’s net deferred tax assets as of December 31, 2015 and 2014 are as follows (in thousands): December 31, 2015 2014 Deferred tax assets: Net operating losses $ 26,676 $ 16,322 Tax credits 2,473 1,139 Accruals 1,117 1,016 Share-based compensation 3,769 2,225 Gross deferred tax assets 34,035 20,702 Valuation allowance (31,185 ) (18,815 ) Net deferred tax assets 2,850 1,887 Deferred tax liability — — Net deferred tax assets $ 2,850 $ 1,887
Summary of Income Tax Contingencies	A reconciliation of the beginning and ending amounts of unrecognized tax benefits is as follows (in thousands): 2015 2014 Gross Unrecognized Tax Benefits at January 1 $ 383 $ 480 Additions for tax positions taken in the current year 678 253 Reductions for tax positions taken in the prior year — (350 ) Gross Unrecognized Tax Benefits at December 31 $ 1,061 $ 383

Quarterly Financial Informati28

Quarterly Financial Information (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Schedule of quarterly financial information	Net loss per share - basic and diluted, for the four quarters of each fiscal year may not sum to the total for the fiscal year because of the different number of shares outstanding during each period (in thousands, except per share data): Quarter First Second Third Fourth Year Ended December 31, 2015 Revenues $ 593 $ 278 $ 429 $ 307 Operating expenses $ 15,622 $ 18,313 $ 23,090 $ 24,519 Net loss $ (15,202 ) $ (18,277 ) $ (22,976 ) $ (24,157 ) Net loss per share - basic $ (0.55 ) $ (0.59 ) $ (0.73 ) $ (0.76 ) Net loss per share - diluted $ (0.55 ) $ (0.59 ) $ (0.73 ) $ (0.76 ) Year Ended December 31, 2014 Revenues $ 32,234 $ 15,121 $ 1,486 $ 2,013 Operating expenses $ 14,215 $ 13,552 $ 14,618 $ 15,118 Net income (loss) $ 17,852 $ 1,290 $ (13,182 ) $ (13,110 ) Net income (loss) per share - basic $ 0.82 $ 0.06 $ (0.48 ) (0.48 ) Net income (loss) per share - diluted $ 0.78 $ 0.06 $ (0.48 ) $ (0.48 )

Organization - Additional Infor

Organization - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit	$ 129,103	$ 48,491
Cash and cash equivalents	$ 370,586	$ 293,579	$ 176,677	$ 124,860

Summary of Significant Accoun30

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Purchased computer software, useful life (in years)	4 years
Minimum [Member]
Property, Plant and Equipment [Line Items]
Machinery and equipment, useful life (in years)	4 years
Maximum [Member]
Property, Plant and Equipment [Line Items]
Machinery and equipment, useful life (in years)	7 years
New Accounting Pronouncement, Early Adoption, Effect [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
Deferred tax assets, net, current		$ (167)
Deferred tax assets, net, noncurrent		$ 167
UNITED STATES
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 3,100
IRELAND
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 800

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Money market funds at carrying value	$ 320.5	$ 262.5

Composition of Certain Balanc32

Composition of Certain Balance Sheet Items - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Composition of Certain Balance Sheet Items [Abstract]
Machinery and equipment	$ 6,210	$ 5,481
Leasehold improvements	2,828	2,214
Purchased computer software	167	137
Property and equipment, gross	9,205	7,832
Less: accumulated depreciation and amortization	(5,343)	(4,711)
Property and equipment, net	$ 3,862	$ 3,121

Composition of Certain Balanc33

Composition of Certain Balance Sheet Items - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Composition of Certain Balance Sheet Items [Abstract]
Depreciation expense	$ 0.8	$ 0.7	$ 0.7

Composition of Certain Balanc34

Composition of Certain Balance Sheet Items - Schedule of Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Composition of Certain Balance Sheet Items [Abstract]
Payroll and related expenses	$ 3,774	$ 3,138
Professional services	325	1,169
Deferred rent	284	172
Other	520	496
Other current liabilities	$ 4,903	$ 4,975

Net Loss Per Ordinary Share - C

Net Loss Per Ordinary Share - Calculation of Basic and Diluted Net Income or Loss Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net income (loss)	$ (24,157)	$ (22,976)	$ (18,277)	$ (15,202)	$ (13,110)	$ (13,182)	$ 1,290	$ 17,852	$ (80,612)	$ (7,150)	$ (40,996)
Denominator:
Weighted-average ordinary shares outstanding (in shares)									30,326	24,672	18,615
Basic and diluted net loss per share (in dollars per share)									$ (2.66)	$ (0.29)	$ (2.20)

Net Loss Per Ordinary Share - O

Net Loss Per Ordinary Share - Ordinary Shares Equivalent Not Included in Diluted Net Loss Per Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Options to purchase ordinary shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Equivalent ordinary shares not included in diluted net loss per share (in shares)	3,142	2,612	1,974

Commitment and Contingencies -

Commitment and Contingencies - Lease Narrative (Details) $ in Thousands, € in Millions	1 Months Ended		12 Months Ended
	Aug. 31, 2015EUR (€)ft²	Jan. 31, 2015	Dec. 31, 2015USD ($)ft²	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Aug. 31, 2015USD ($)
Commitments and Contingencies Disclosure [Abstract]
Expiration of lease			Nov. 30, 2020
Operating leases, rent expense			$ 2,100
Area of rentable square feet (in sqft) | ft²			50,400
Sublease leases, term (in years)		3 years
Future Minimum Sublease Rentals Year Two			$ 316
Future minimum sublease rentals, year one			523
Future minimum sublease rentals year two			0
Rent expense			1,900	$ 1,800	$ 1,300
Loss Contingencies [Line Items]
Future minimum payments due			12,433
Dublin, Ireland [Member]
Loss Contingencies [Line Items]
Operating leases, area of office space (in sqft) | ft²	6,258
Operating leases, term of contract (in years)	10 years
Operating leases, rent review term (in years)	5 years
Future minimum payments due	€ 2		$ 2,100			$ 2,200

Commitment and Contingencies 38

Commitment and Contingencies - Schedule of Operating Lease Commitment (Details) $ in Thousands	Dec. 31, 2015USD ($)
Operating Lease
2,016	$ 2,126
2,017	2,242
2,018	2,322
2,019	2,406
2,020	2,283
Thereafter	1,054
Total future minimum lease payments	12,433
Sublease Rental
2,016	(523)
2,017	(316)
2,018	0
2,019	0
2,020	0
Thereafter	0
Total	$ (839)

Commitment and Contingencies 39

Commitment and Contingencies - Commitment Narrative (Details) $ in Thousands	Dec. 31, 2015USD ($)
Long-term Purchase Commitment [Line Items]
Purchase obligation	$ 7,081
Contractual obligation under license agreements	1,507
Accrued current liabilities [Member]
Long-term Purchase Commitment [Line Items]
Commitment to suppliers included in accrued current liabilities	3,100
Contractual obligations under license agreements included in accrued current liabilities	100
License agreements [Member]
Long-term Purchase Commitment [Line Items]
Contractual obligation under license agreements	$ 1,500

Commitment and Contingencies Co

Commitment and Contingencies Commitment and Contingencies - Contractual Obligations (Details) $ in Thousands	Dec. 31, 2015USD ($)
Purchase Obligations
Total	$ 7,081
2,016	7,028
2,017	44
2,018	9
2,019	0
2,020	0
Thereafter	0
Contractual obligations under license agreements
Total	1,507
2,016	233	[1]
2,017	123
2,018	123
2,019	123
2,020	93
Thereafter	812
Total
Total	8,588
2,016	7,261
2,017	167
2,018	132
2,019	123
2,020	93
Thereafter	$ 812
[1]	Excludes future obligations pursuant to the cost-sharing arrangement under the Company's License Agreement with Roche. Amounts of such obligations, if any, cannot be determined at this time.

Roche License Agreements (Detai

Roche License Agreements (Details) - USD ($)	1 Months Ended		12 Months Ended
	May. 31, 2014	Feb. 28, 2014	Dec. 31, 2015	Dec. 31, 2014
Collaborative Arrangement [Member]
License Agreement [Line Items]
Portion of revenue and expenses attributable to company, percentage			30.00%
Roche [Member] | Collaborative Arrangement [Member]
License Agreement [Line Items]
Milestone payment received, clinical milestone	$ 15,000,000
Potential payment upon achievement of development, regulatory and various first commercial sales milestones			$ 380,000,000
Potential payment for achievement of non U.S.commercial sales milestones			$ 175,000,000
Cost allocation, percentage			100.00%
Portion of revenue and expenses attributable to company, percentage			70.00%
Potential alternative commercial sales milestone payment			$ 155,000,000
Multiple element consideration, relative sales price method, amt allocable to license			35,600,000
Cost sharing payments recognized as research and development expense			2,900,000	$ 1,400,000
Development reimbursement			5,100,000	6,000,000
Milestone payment recognized as collaboration revenue				13,300,000
Milestone payment used to offset research and development expense				1,700,000
Roche [Member] | License [Member]
License Agreement [Line Items]
Upfront payment pursuant to license agreement		$ 30,000,000
Roche [Member] | Research Reimbursement [Member]
License Agreement [Line Items]
Collaboration service revenue, research services			1,400,000	1,700,000
Roche [Member] | Development Costs Reimbursement [Member]
License Agreement [Line Items]
Development reimbursement recognized as collaboration license revenue			200,000	5,300,000
Development reimbursement recognized as offset to research and development expense			$ 4,900,000	$ 700,000

Shareholders' Equity Ordinary S

Shareholders' Equity Ordinary Shares/Euro Deferred Shares (Details)	12 Months Ended
	Dec. 31, 2015vote$ / sharesshares	Dec. 31, 2015€ / sharesshares	Dec. 31, 2014$ / sharesshares	Dec. 31, 2014€ / sharesshares
Equity [Abstract]
Ordinary shares, number of authorized shares	100,000,000	100,000,000	100,000,000	100,000,000
Ordinary shares, par value (in dollars per share) | $ / shares	$ 0.01		$ 0.01
Ordinary shares, number of issued shares	31,744,102	31,744,102	27,388,005	27,388,005
Ordinary shares, number of outstanding shares	31,744,102	31,744,102	27,388,005	27,388,005
Votes per share | vote	1
Euro deferred shares, number of shares authorized	10,000	10,000	10,000	10,000
Euro deferred shares, nominal value (in euros per share) | € / shares		€ 22		€ 22
Euro deferred shares, number of outstanding shares	0	0	0	0

Shareholders' Equity Issuance o

Shareholders' Equity Issuance of Ordinary Shares (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended					12 Months Ended
	Apr. 30, 2015	Jul. 31, 2014	Jun. 30, 2014	Feb. 28, 2014	Oct. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Shareholders Equity [Line Items]
Issuance of ordinary shares in public offering, net of issuance cost		$ 117,429				$ 131,481	$ 117,429	$ 84,453
Public offering costs								(7,400)
Underwritten Public Offering [Member]
Shareholders Equity [Line Items]
Ordinary shares sold (in dollars per share)					$ 22
Public offering costs						(8,900)	(5,500)
Net proceeds from issuance of ordinary shares in underwritten public offering			$ 102,500
Underwritten Public And Over-Allotment Offering [Member]
Shareholders Equity [Line Items]
Ordinary shares sold (in dollars per share)	$ 37	$ 22.500
Issuance of ordinary shares in public offering, net of issuance cost					$ 84,500
Net proceeds from issuance of ordinary shares in underwritten public offering	$ 131,500
Ordinary Shares
Shareholders Equity [Line Items]
Issuance of ordinary shares in public offering, net of issuance cost						$ 38	$ 54	$ 42
Ordinary Shares | Janssen [Member]
Shareholders Equity [Line Items]
Shares sold pursuant to underwritten public offering					2,619,421
Ordinary Shares | Perrigo [Member]
Shareholders Equity [Line Items]
Shares sold pursuant to underwritten public offering				3,182,253
Share price				$ 26
Ordinary Shares | Underwritten Public And Over-Allotment Offering [Member]
Shareholders Equity [Line Items]
Ordinary shares sold pursuant to underwritten public offering (in shares)	3,795,000	5,462,500			4,177,079
Underwritten Public Offering [Member] | Ordinary Shares
Shareholders Equity [Line Items]
Shares sold pursuant to underwritten public offering					6,796,500
Over-Allotment Option [Member] | Underwritten Public Offering [Member]
Shareholders Equity [Line Items]
Net proceeds from issuance of ordinary shares in underwritten public offering		$ 14,900

Share-Based Compensation - Addi

Share-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Common Stock [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Tax benefit realized from exercise of stock options	$ 1.7	$ 1	$ 0.4
Intrinsic value of options exercised	$ 25.5	$ 1.5	$ 0
Amended and Restated 2012 Long Term Incentive Plan [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Grant period (in years)	10 years
Number of stock options granted to employees (in shares)	1,155,300	720,500	1,978,000
Vesting period (in years)	4 years
Authorized shares for issuance (in shares)	5,550,000
Number of shares available for grant (in shares)	1,777,295
Options outstanding (in shares)	3,142,364	2,612,080
Weighted average exercise price (in dollars per share)	$ 21.36	$ 13.13
Share-based compensation not yet recognized, stock options	$ 31.2
Share-based compensation not yet recognized, period for recognition (in years)	2 years 5 months 16 days

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Share-Based Compensation Expense (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 10,414	$ 5,597	$ 3,128
Consultant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense on option		42	108	320
Research and development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	[1]	4,301	2,270	980
General and administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 6,113	$ 3,327	$ 2,148
[1]	Includes $42,000, $108,000 and $320,000 for the years ended December 31, 2015, 2014 and 2013, respectively, of share-based compensation expense related to options granted to a consultant.

Share-Based Compensation - Fair

Share-Based Compensation - Fair Value of Options Granted (Detail) - Amended and Restated 2012 Long Term Incentive Plan [Member] - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	76.30%	84.40%	84.00%
Risk-free interest rate	1.70%	1.80%	1.20%
Expected dividend yield	0.00%	0.00%	0.00%
Expected life (in years)	6 years	6 years	6 years
Weighted average fair value (in dollars per share)	$ 23.12	$ 19.97	$ 5.22

Share-based Compensation - Shar

Share-based Compensation - Share-based Compensation Plan - Option Activity (Detail) - Amended and Restated 2012 Long Term Incentive Plan [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options, outstanding beginning balance	2,612,080
Options, granted	1,155,300	720,500	1,978,000
Options exercised	(561,097)
Options, canceled	(63,919)
Options, outstanding ending balance	3,142,364	2,612,080
Options, vested and expected to vest ending balance	3,053,060
Options, vested outstanding number	1,169,118
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted average exercise price, outstanding beginning balance (in dollars per share)	$ 13.13
Weighted average exercise price, granted (in dollars per share)	34.58
Weighted average exercise price, exercised (in dollars per share)	10.06
Weighted average exercise price, canceled (in dollars per share)	23.27
Weighted average exercise price, outstanding ending balance (in dollars per share)	21.36	$ 13.13
Weighted average exercise price, vested and expected to vest ending balance (in dollars per share)	21.08
Weighted average exercise price, options vested (in dollars per share)	$ 11.67
Weighted average remaining contractual life, outstanding ending balance (in years)	8 years 1 month 21 days	8 years 5 months 12 days
Weighted average remaining contractual life, vested and expected to vest ending balance (in years)	8 years 1 month 13 days
Weighted average remaining contractual term, vested outstanding (in years)	7 years 4 months 10 days
Aggregate intrinsic value, outstanding beginning balance	$ 25,298
Aggregate intrinsic value, outstanding ending balance	146,917	$ 25,298
Aggregate intrinsic value, vested and expected to vest ending balance	143,578
Aggregate intrinsic value, options vested	$ 65,983

Share-Based Compensation - Exer

Share-Based Compensation - Exercise Price (Details) - Amended and Restated 2012 Long Term Incentive Plan [Member] - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)	$ 6.03
Exercise price, upper limit (in dollars per share)	$ 70.17
Number of options outstanding (in shares)	3,142,364
Options outstanding, weighted-average remaining contractual life (in years)	8 years 1 month 21 days	8 years 5 months 12 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 21.36
Options exercisable, number of options (in shares)	1,169,118
Options exercisable, weighted average exercise price (in dollars per share)	$ 11.67
$6.03 - $6.03 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)	6.03
Exercise price, upper limit (in dollars per share)	$ 6.03
Number of options outstanding (in shares)	383,334
Options outstanding, weighted-average remaining contractual life (in years)	7 years 29 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 6.03
Options exercisable, number of options (in shares)	261,459
Options exercisable, weighted average exercise price (in dollars per share)	$ 6.03
$6.41 - $6.41 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)	6.41
Exercise price, upper limit (in dollars per share)	$ 6.41
Number of options outstanding (in shares)	591,665
Options outstanding, weighted-average remaining contractual life (in years)	7 years 29 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 6.41
Options exercisable, number of options (in shares)	429,024
Options exercisable, weighted average exercise price (in dollars per share)	$ 6.41
$6.65 - $16.42 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)	6.65
Exercise price, upper limit (in dollars per share)	$ 16.42
Number of options outstanding (in shares)	340,250
Options outstanding, weighted-average remaining contractual life (in years)	7 years 3 months 15 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 8.01
Options exercisable, number of options (in shares)	214,946
Options exercisable, weighted average exercise price (in dollars per share)	$ 8.51
$17.36 - $26.47 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)	17.36
Exercise price, upper limit (in dollars per share)	$ 26.47
Number of options outstanding (in shares)	230,838
Options outstanding, weighted-average remaining contractual life (in years)	8 years 3 months 26 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 21.53
Options exercisable, number of options (in shares)	59,458
Options exercisable, weighted average exercise price (in dollars per share)	$ 21.59
$27.81 - $27.81 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)	27.81
Exercise price, upper limit (in dollars per share)	$ 27.81
Number of options outstanding (in shares)	754,550
Options outstanding, weighted-average remaining contractual life (in years)	9 years 2 months 9 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 27.81
Options exercisable, number of options (in shares)	0
Options exercisable, weighted average exercise price (in dollars per share)	$ 0
$29.52 - $29.52 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)	29.52
Exercise price, upper limit (in dollars per share)	$ 29.52
Number of options outstanding (in shares)	14,063
Options outstanding, weighted-average remaining contractual life (in years)	8 years 1 month 2 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 29.52
Options exercisable, number of options (in shares)	1,042
Options exercisable, weighted average exercise price (in dollars per share)	$ 29.52
$29.81 - $29.81 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)	29.81
Exercise price, upper limit (in dollars per share)	$ 29.81
Number of options outstanding (in shares)	437,914
Options outstanding, weighted-average remaining contractual life (in years)	8 years 1 month 2 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 29.81
Options exercisable, number of options (in shares)	186,731
Options exercisable, weighted average exercise price (in dollars per share)	$ 29.81
$33.65 - $55.05 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)	33.65
Exercise price, upper limit (in dollars per share)	$ 55.05
Number of options outstanding (in shares)	316,000
Options outstanding, weighted-average remaining contractual life (in years)	9 years 5 months 1 day
Options outstanding, weighted average exercise price (in dollars per share)	$ 43.89
Options exercisable, number of options (in shares)	16,458
Options exercisable, weighted average exercise price (in dollars per share)	$ 37.02
$67.64 - $67.64 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)	67.64
Exercise price, upper limit (in dollars per share)	$ 67.64
Number of options outstanding (in shares)	72,500
Options outstanding, weighted-average remaining contractual life (in years)	9 years 7 months 2 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 67.64
Options exercisable, number of options (in shares)	0
Options exercisable, weighted average exercise price (in dollars per share)	$ 0
$70.17 - $70.17 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, lower limit (in dollars per share)	70.17
Exercise price, upper limit (in dollars per share)	$ 70.17
Number of options outstanding (in shares)	1,250
Options outstanding, weighted-average remaining contractual life (in years)	9 years 11 months 1 day
Options outstanding, weighted average exercise price (in dollars per share)	$ 70.17
Options exercisable, number of options (in shares)	0
Options exercisable, weighted average exercise price (in dollars per share)	$ 0

Income Taxes - Loss Before Prov

Income Taxes - Loss Before Provision for Income Taxes by Region (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Ireland	$ (83,009)	$ (9,714)	$ (42,523)
U.S.	3,098	3,375	1,942
Loss before provision for income taxes	$ (79,911)	$ (6,339)	$ (40,581)

Income Taxes - Components of th

Income Taxes - Components of the Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current:
U.S. Federal	$ 1,663	$ 1,855	$ 958
U.S. State	1	1	(5)
Ireland	0	0	0
Total current provision	1,664	1,856	953
Deferred:
U.S. Federal	(963)	(1,045)	(538)
U.S. State	0	0	0
Ireland	0	0	0
Total deferred benefit	(963)	(1,045)	(538)
Provision for income taxes	$ 701	$ 811	$ 415

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Taxes [Line Items]
Excess tax benefit from share-based award exercises	$ 3,855	$ 242	$ 0
Effective income tax rate, Ireland statutory, percent	12.50%	12.50%	12.50%
Valuation allowance	$ (31,185)	$ (18,815)
Net deferred tax assets	2,850	1,887
Increase in valuation allowance	12,400
Tax credits carryforwards	$ 2,473	$ 1,139
Ireland [Member]
Income Taxes [Line Items]
Effective income tax rate, Ireland statutory, percent	12.50%
United States [Member]
Income Taxes [Line Items]
Operating loss carryforwards	$ 700
Tax credits carryforwards	3,900
Tax deductions from equity compensation, not included in deferred tax assets	3,500
Cumulative unremitted earnings	$ 6,500
Potential U.S. withholding tax rate	5.00%
California [Member]
Income Taxes [Line Items]
Operating loss carryforwards	$ 36,400
Tax credits carryforwards	2,500
Trading Loss Carryover [Member] | Ireland [Member]
Income Taxes [Line Items]
Operating loss carryforwards	179,700
Non-Trading Loss Carryover [Member] [Domain] | Ireland [Member]
Income Taxes [Line Items]
Operating loss carryforwards	$ 8,000

Income Taxes - Reconciliation B

Income Taxes - Reconciliation Between Income Taxes Computed at the Standard Irish Statutory Tax Rate and the Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Effective income tax rate, Ireland statutory, percent	12.50%	12.50%	12.50%
Taxes at the Irish statutory tax rate of 12.5%	$ (9,989)	$ (792)	$ (5,073)
Income tax at rates other than applicable statutory rate	446	705	(3,169)
Change in valuation allowance	12,594	2,444	10,365
Share-based payments	214	74	164
Tax credits	(2,712)	(1,643)	(1,921)
Other	148	23	49
Provision for income taxes	$ 701	$ 811	$ 415

Income Taxes - Significant Comp

Income Taxes - Significant Components of Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Net operating losses	$ 26,676	$ 16,322
Tax credits	2,473	1,139
Accruals	1,117	1,016
Share-based compensation	3,769	2,225
Gross deferred tax assets	34,035	20,702
Valuation allowance	(31,185)	(18,815)
Net deferred tax assets	2,850	1,887
Deferred tax liability	0	0
Net deferred tax assets	$ 2,850	$ 1,887

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Gross Unrecognized Tax Benefits at January 1	$ 383	$ 480
Additions for tax positions taken in the current year	678	253
Reductions for tax positions taken in the prior year	0	(350)
Gross Unrecognized Tax Benefits at December 31	$ 1,061	$ 383

Employee Retirement Plan (Detai

Employee Retirement Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Contribution Plan Disclosure [Line Items]
Maximum annual contributions per employee, percent	100.00%
Prothena 401K Retirement Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution, percent of employees' gross pay	3.00%
Recorded expense for matching contributions	$ 400	$ 300	$ 300
Irish Defined Contribution Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Maximum annual contributions per employee, percent	7.50%
Recorded expense for matching contributions	$ 51	$ 19

Related Parties (Detail)

Related Parties (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Feb. 28, 2014	Dec. 31, 2012	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 20, 2012
Related Party Transaction [Line Items]
Revenue from related parties			$ 0	$ 534,000	$ 676,000
Transitional Services Agreement [Member]
Related Party Transaction [Line Items]
Additional percentage of employee service payment		40.00%
TSA expenses recognized			$ 0	0	500,000
Transitional Services Agreement [Member] | Research and development [Member]
Related Party Transaction [Line Items]
TSA expenses recognized					100,000
Transitional Services Agreement [Member] | General and administrative [Member]
Related Party Transaction [Line Items]
TSA expenses recognized					400,000
R&D Services Agreement [Member]
Related Party Transaction [Line Items]
Revenue from related parties				$ 534,000	$ 676,000
Research and development service agreement, Term (in years)		2 years
Fixed service fee, annual rate		$ 500,000
Fixed service fee per employee, annual rate		250,000
Variable service fee, annual rate		$ 250,000
Percentage of mark-up applied to fixed and variable charge		10.00%
Elan [Member]
Related Party Transaction [Line Items]
Percentage of ownership interest acquired						18.00%
Issuance of ordinary shares to Elan, shares					3,182,253
Elan [Member] | Transitional Services Agreement [Member]
Related Party Transaction [Line Items]
Additional percentage of employee service payment		40.00%
Perrigo [Member] | Ordinary Shares
Related Party Transaction [Line Items]
Shares sold pursuant to underwritten public offering	3,182,253
Share price	$ 26

Quarterly Financial Informati57

Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 307	$ 429	$ 278	$ 593	$ 2,013	$ 1,486	$ 15,121	$ 32,234	$ 1,607	$ 50,854	$ 676
Operating expenses	24,519	23,090	18,313	15,622	15,118	14,618	13,552	14,215	81,544	57,503	41,103
Net income (loss)	$ (24,157)	$ (22,976)	$ (18,277)	$ (15,202)	$ (13,110)	$ (13,182)	$ 1,290	$ 17,852	$ (80,612)	$ (7,150)	$ (40,996)
Net Income (Loss) Per Share, Basic	$ (0.76)	$ (0.73)	$ (0.59)	$ (0.55)	$ (0.48)	$ (0.48)	$ 0.06	$ 0.82
Net Income (Loss) Per Share, Diluted	$ (0.76)	$ (0.73)	$ (0.59)	$ (0.55)	$ (0.48)	$ (0.48)	$ 0.06	$ 0.78

Subsequent Events (Details)

Subsequent Events (Details) - Underwritten Public And Over-Allotment Offering [Member] - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Jan. 31, 2016	Apr. 30, 2015	Jul. 31, 2014	Oct. 31, 2013
Subsequent Event [Line Items]
Ordinary shares sold (in dollars per share)		$ 37	$ 22.500
Net proceeds from issuance of ordinary shares in underwritten public offering		$ 131.5
Subsequent Event [Member]
Subsequent Event [Line Items]
Ordinary shares sold (in dollars per share)	$ 53
Net proceeds from issuance of ordinary shares in underwritten public offering	$ 128.6
Ordinary Shares
Subsequent Event [Line Items]
Ordinary shares sold pursuant to underwritten public offering (in shares)		3,795,000	5,462,500	4,177,079
Ordinary Shares | Subsequent Event [Member]
Subsequent Event [Line Items]
Ordinary shares sold pursuant to underwritten public offering (in shares)	2,587,500