Prothena (PRTA) 10-K 7 Mar 14 2013 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2013	Mar. 03, 2014	Jun. 28, 2013
Document Document And Entity Information [Abstract]	'	'	'
Document Type	'10-K	'	'
Amendment Flag	'false	'	'
Document Period End Date	31-Dec-13	'	'
Document Fiscal Year Focus	'2013	'	'
Document Fiscal Period Focus	'FY	'	'
Trading Symbol	'PRTA	'	'
Entity Registrant Name	'Prothena Corp plc	'	'
Entity Central Index Key	'0001559053	'	'
Current Fiscal Year End Date	'--12-31	'	'
Entity Filer Category	'Accelerated Filer	'	'
Entity Ordinary Shares Outstanding	'	21,902,937	'
Entity Well-known Seasoned Issuer	'No	'	'
Entity Current Reporting Status	'Yes	'	'
Entity Voluntary Filers	'No	'	'
Entity Public Float	'	'	$187.10

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Current assets:	'	'
Cash and cash equivalents	$176,677	$124,860
Receivable from related party	58	223
Deferred tax assets	81	73
Prepaid expenses and other current assets	1,406	685
Total current assets	178,222	125,841
Non-current assets:	'	'
Property and equipment, net	3,372	3,442
Deferred tax assets	816	0
Total non-current assets	4,188	3,442
Total assets	182,410	129,283
Current liabilities:	'	'
Accounts payable	1,790	0
Accrued research and development	1,542	47
Income taxes payable	184	27
Other current liabilities	3,890	1,670
Total current liabilities	7,406	1,744
Non-current liabilities:	'	'
Deferred rent	1,734	1,055
Total liabilities	9,140	2,799
Commitments and contingencies (Note 6)	'	'
Shareholders’ equity:	'	'
Euro deferred shares, €22 nominal value: Authorized shares - 10,000 at December 31, 2013 and December 31, 2012 Issued and outstanding shares - none at December 31, 2013 and December 31, 2012	0	0
Ordinary shares, $0.01 par value: Authorized shares - 100,000,000 at December 31, 2013 and December 31, 2012 Issued and outstanding shares - 21,856,261 at December 31, 2013 and 17,679,182 at December 31, 2012	219	177
Additional paid-in capital	214,392	126,652
Accumulated deficit	-41,341	-345
Total shareholders’ equity	173,270	126,484
Total liabilities and shareholders’ equity	$182,410	$129,283

Condensed_Consolidated_Balance1

Condensed Consolidated Balance Sheets (Parenthetical)	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012
	USD ($)	EUR (€)	USD ($)	EUR (€)
Statement of Financial Position [Abstract]	'	'	'	'
Euro deferred shares, nominal value (in euros per share)	'	€ 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)	$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	21,856,261	21,856,261	17,679,182	17,679,182
Ordinary shares, number of outstanding shares	21,856,261	21,856,261	17,679,182	17,679,182

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Operations (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Revenues—related party	$676	$2,658	$507
Operating expenses:	'	'	'
Research and development	26,052	34,139	24,172
General and administrative	15,051	9,929	5,579
Total operating expenses	41,103	44,068	29,751
Loss from operations	-40,427	-41,410	-29,244
Other income (expense):	'	'	'
Interest income	71	5	0
Other income (expense), net	-225	0	0
Total other income (expense)	-154	5	0
Loss before income taxes	-40,581	-41,405	-29,244
Provision for income taxes	415	6	426
Net loss	($40,996)	($41,411)	($29,670)
Basic and diluted net loss per share (in dollars per share)	($2.20)	($2.84)	($2.05)
Shares used to compute basic and diluted net loss per share (in shares)	18,615	14,593	14,497

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating activities	'	'	'
Net loss	($40,996)	($41,411)	($29,670)
Adjustments to reconcile net loss to cash used in operating activities:	'	'	'
Depreciation and amortization	660	468	391
Share-based compensation	3,128	6,098	2,972
Deferred income taxes	-538	0	0
Gain on disposal of fixed asset	-29	0	0
Changes in operating assets and liabilities:	'	'	'
Receivable from related party	165	-223	-146
Other assets	-721	-467	0
Accounts payable, accruals and other liabilities	6,233	-6,537	6,756
Net cash used in operating activities	-32,098	-42,072	-19,697
Investing activities	'	'	'
Purchases of property and equipment	-564	-1,301	-595
Proceeds from disposal of fixed asset	29	0	0
Net cash used in investing activities	-535	-1,301	-595
Financing activities	'	'	'
Proceeds from funding provided by Elan	0	145,233	20,292
Repayment of funding provided by Elan	0	-3,000	0
Post separation adjustments to the funding provided by Elan	-84	0	0
Proceeds from issuance of ordinary shares to Elan	0	26,000	0
Proceeds from issuance of ordinary shares in public offering, net	84,534	0	0
Net cash provided by financing activities	84,450	168,233	20,292
Net increase in cash and cash equivalents	51,817	124,860	0
Cash and cash equivalents, beginning of the year	124,860	0	0
Cash and cash equivalents, end of the period	176,677	124,860	0
Supplemental disclosures of cash flow information	'	'	'
Cash paid for income taxes, net of refunds	796	0	0
Supplemental disclosures of non cash investing and financing activities	'	'	'
Acquisition of property and equipment under accounts payable and accrued liabilities	26	0	0
Accrued deferred offering costs	$82	$0	$0

Consolidated_Statement_of_Shar

Consolidated Statement of Shareholders' Equity Statement (USD $)	Total	Ordinary Shares	Additional Paid-in Capital	Accumulated Deficit	Parent Company Equity
Beginning balance, value at Dec. 31, 2010	($30,000)	$0	$0	$0	($30,000)
Beginning balance, shares at Dec. 31, 2010	'	0	'	'	'
Increase (Decrease) in Stockholders' Equity [Roll Forward]	'	'	'	'	'
Share-based compensation	2,972,000	'	'	'	2,972,000
Net funding provided by Elan	20,292,000	'	'	'	20,292,000
Net loss	-29,670,000	'	'	'	-29,670,000
Ending balance, value at Dec. 31, 2011	-6,436,000	0	0	0	-6,436,000
Beginning balance, shares at Dec. 31, 2011	'	0	'	'	'
Increase (Decrease) in Stockholders' Equity [Roll Forward]	'	'	'	'	'
Number of ordinary shares issued, New Issues	'	14,496,929	'	'	'
Net proceeds received for the sale of shares	'	145,000	100,684,000	'	-100,829,000
Issuance of ordinary shares to Elan, shares	'	3,182,253	'	'	'
Issuance of ordinary shares to Elan, value	26,000,000	32,000	25,968,000	'	'
Share-based compensation	6,098,000	'	'	'	6,098,000
Net funding provided by Elan	142,233,000	'	'	'	142,233,000
Net loss	-41,411,000	'	'	-345,000	-41,066,000
Ending balance, value at Dec. 31, 2012	126,484,000	177,000	126,652,000	-345,000	0
Ending balance, shares at Dec. 31, 2012	'	17,679,182	'	'	'
Increase (Decrease) in Stockholders' Equity [Roll Forward]	'	'	'	'	'
Number of ordinary shares issued, New Issues	'	4,177,079	'	'	'
Issuance costs	7,400,000	'	'	'	'
Net proceeds received for the sale of shares	84,453,000	42,000	84,411,000	'	'
Share-based compensation	3,128,000	'	3,128,000	'	'
Post separation adjustment to the funding provided by Elan	201,000	'	201,000	'	'
Net loss	-40,996,000	'	'	-40,996,000	'
Ending balance, value at Dec. 31, 2013	$173,270,000	$219,000	$214,392,000	($41,341,000)	$0
Ending balance, shares at Dec. 31, 2013	'	21,856,261	'	'	'

Organization

Organization	12 Months Ended
	Dec. 31, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'
Organization	'
	Organization
	Description of Business
	Prothena Corporation plc and subsidiaries (“Prothena” or the “Company”) is a clinical stage biotechnology company focused on the discovery, development and commercialization of novel antibodies for the potential treatment of diseases that involve protein misfolding or cell adhesion. The Company is focused on therapeutic monoclonal antibodies directed specifically to disease causing proteins. The Company's antibody-based product candidates target a number of potential indications including AL and AA forms of amyloidosis (NEOD001), Parkinson’s disease and other related synucleinopathies (PRX002) and novel cell adhesion targets involved in inflammatory diseases and cancers (PRX003). The Company initiated a Phase 1 clinical trial for NEOD001, with successful first patient dosing in April 2013. The Phase 1 clinical trial of NEOD001 is evaluating its safety and tolerability in patients with AL amyloidosis. The Company also plans to initiate Phase 1 clinical trials for PRX002 and PRX003 in 2014 and 2015, respectively. The Company's strategy is to identify antibody candidates for clinical development by applying its extensive expertise in generating novel therapeutic antibodies and working with collaborators having expertise in specific animal models of disease.
	The Company is a public limited company formed under the laws of Ireland. The Company separated from Elan Corporation Limited, formerly Elan Corporation, plc (“Elan”), which was subsequently acquired by Perrigo Company plc (“Perrigo”), on December 20, 2012. Prothena's business consists of a substantial portion of Elan's former drug discovery business platform, including Neotope Biosciences Limited and its wholly owned subsidiaries Onclave Therapeutics Limited and Prothena Biosciences Inc. (which for the period prior to separation and distribution are referred to herein as the “Prothena Business”). Prior to December 21, 2012, the Prothena Business operated as part of Elan and not as a separate stand-alone entity. After the separation from Elan, and the related distribution of the Company's ordinary shares to Elan’s shareholders (the “Separation and Distribution”), 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.
	In connection with the Separation and Distribution, Elan invested total cash in the Company of $125.0 million in return for 18% of the Company's outstanding ordinary shares (as calculated immediately following the consummation of such subscription) that a wholly-owned subsidiary of Elan subscribed for immediately following the Separation and Distribution.
	Liquidity and Business Risks
	As of December 31, 2013, the Company had an accumulated deficit of $41.3 million and cash and cash equivalents of $176.7 million, respectively. Based on the Company's business plans, management believes that the Company's cash and cash equivalents at December 31, 2013 are sufficient to meet its obligations for at least the next respective 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 operating activities primarily through its current cash and cash equivalents, and to the extent necessary, through proceeds from public or private equity or debt financings, loans and collaborative agreements with corporate partners or other arrangements. In October 2013, the Company sold an aggregate of 4,177,079 ordinary shares for net proceeds of approximately $84.5 million, after deducting the underwriting discount and estimated offering expenses, in an underwritten public offering.
	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 new products; 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.
	The Company is dependent on Boehringer Ingelheim to manufacture clinical supplies for its therapeutic antibody programs. An inability to obtain product supply could have a material adverse impact on the Company's business, financial condition and results of operations.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2013
Accounting Policies [Abstract]	'
Summary of Significant Accounting Policies	'
	Summary of Significant Accounting Policies
	Basis of Preparation and Presentation of Financial Information
	The Prothena Business historically operated as part of Elan and not as a separate stand-alone entity. Prior to the separation on December 20, 2012, the consolidated financial statements of Prothena have been prepared on a “carve-out” basis from the consolidated financial statements of Elan to represent the financial position and performance of Prothena as if the Company had existed on a stand-alone basis and as if Financial Accounting Standards Board (“FASB”) Accounting Standard Codification (“ASC”) Topic 810, “Consolidation” (“ASC 810”) had been applied throughout. The accompanying Consolidated Financial Statements prior to December 21, 2012 include only those assets and liabilities that management has determined are specifically identifiable to Prothena and allocations of direct costs and indirect costs attributable to the Company's operations. The indirect costs included in the Company's Consolidated Financial Statements relate to certain centralized support functions that were provided by Elan. All intragroup transactions within the Prothena Business have been eliminated in the Consolidated Financial Statements and are not disclosed.
	These Consolidated Financial Statements have been prepared in accordance with the accounting principles generally accepted in the United States of America (“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. The financial information for all periods prior to the Separation and Distribution were prepared by aggregating financial information from the components of Prothena as described above. All financial information presented after December 20, 2012 includes the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
	Prior to December 21, 2012, the centralized support functions provided to the Company by Elan included, but were not limited to, accounting, information technology, taxation, legal, corporate strategy, investor relations, corporate governance and other professional services, employee benefit administration, including equity award and pension services, and cash and treasury management. Centralized support costs allocated to the Prothena business for the years ended December 31, 2012 and 2011 were $7.7 million and $4.0 million, respectively. These costs were allocated to the Company for the purposes of preparing the Consolidated Financial Statements based on estimated usage of the resources by the Prothena Business. The estimated usage of the central support resources allocated to the Prothena Business was determined by estimating its portion of the most appropriate driver for each category of central support costs such as headcount or labor hours, depending on the nature of the costs. The Company believes that such allocations were made on a reasonable basis, but may not necessarily be indicative of the costs that would have been incurred if the Prothena Business had operated on a standalone basis.
	Elan used a centralized approach to manage substantially all of its liquid resources and to finance its operations and, as a result, no separate cash accounts for Prothena were historically maintained, and debt and liquid resources maintained at the Elan group level are not included in the accompanying Consolidated Financial Statements prior to the separation. Elan historically funded all of Prothena's operating and capital resource requirements. The parent company equity balance in the Consolidated Financial Statements constitutes Elan's investment in Prothena and represents the excess of total liabilities over total assets (or excess of total assets over total liabilities), including the netting of intercompany funding balances between Prothena and Elan. Changes in parent company equity represent Elan's net investment in Prothena, after giving effect to its net loss, contributions from Elan in the form of share-based compensation to Prothena's employees and net funding provided by Elan.
	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. 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, 2013, 2012, and 2011.
	Revenue
	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. Advance payments received in excess of amounts earned are classified as deferred revenue until earned. Up-front fees are deferred and amortized to the income statement over the performance period. The performance period is the period over which the Company expects to provide services as determined by the contract provisions.
	The Company recognizes revenue from the delivery of research and development services. Revenue is recognized when all of the following criteria are met: persuasive evidence of an arrangement exists, delivery has occurred or services have been rendered and the contractually specified acceptance criteria have been met, the fee is fixed or determinable, and collectibility is reasonably assured. If sales arrangements contain multiple elements, the Company evaluates whether the components of each arrangement represent separate units of accounting.
	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 our Consolidated Financial Statements to the actual services received and efforts expended. As such, expense accruals related to clinical trials are recognized based on our 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 up-front 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).
	Total share-based compensation expense recorded in the Consolidated Financial Statements for the years ended December 31, 2012 and 2011 was allocated to the Company based on awards from Elan equity plans granted to Elan employees who have, directly or indirectly, provided services to the Company.
	With respect to Elan options and RSUs held by Elan employees that became employees of Prothena effective upon the Separation and Distribution:
	•	unvested Elan options and RSUs that would otherwise have vested within twelve months following the effective date of the Separation and Distribution vested immediately upon the Separation and Distribution, with the RSUs (which by their terms are settled upon vesting) settled in Elan ordinary shares or Elan ADSs in accordance with their terms;
	•	other unvested Elan options and RSUs were forfeited; and
	•	all vested Elan options (including options the vesting of which were accelerated as described above) will be required to be exercised for Elan ordinary shares or Elan ADSs within twelve months of the effective date of the Separation and Distribution, or will be forfeited.
	However, for Elan employees who are aged 55 or over with at least five years of service and who became employees of the Company, unvested Elan options and RSUs became fully vested and exercisable upon the Separation and Distribution, with the RSUs (which, by their terms, are settled upon vesting) settled in Elan ordinary shares or Elan ADSs in accordance with their terms, and with Elan options being exercisable for one year following the Separation and Distribution. Similarly, unvested Elan options and RSUs held by Dr. Schenk, became fully vested and exercisable upon the Separation and Distribution, with the RSUs (which, by their terms are settled upon vesting) settled in Elan ordinary shares or Elan ADSs in accordance with their terms, and with Elan options being exercisable for two years following the Separation and Distribution.
	The Company did not recognize any expense after December 20, 2012 in relation to the existing Elan equity-based awards as the Company’s employees are not required to provide service after the Separation and Distribution in order to receive the benefits of the awards. The share-based compensation expense relating to the changes described above is a non-recurring charge that is directly attributable to Elan as part of the Separation and Distribution of the Prothena Business, therefore it was not recorded in the Company’s Consolidated Financial Statements after December 20, 2012.
	Income Taxes
	Subsequent to the separation from Elan, Prothena began to file 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. Prior to the separation from Elan, income taxes as presented in the Consolidated Financial Statements represented current and deferred income taxes of Elan attributed to the Company in a manner that is systematic, rational and consistent with the asset and liability method prescribed by the accounting guidance for income taxes. The Company's income tax provision prior to the separation from Elan was prepared under the “separate return method.” The separate return method applies the accounting guidance for income taxes to the standalone Consolidated Financial Statements as if the Company was a separate taxpayer and a standalone enterprise.
	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.
	Significant 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.
	Comprehensive Loss
	Comprehensive loss is comprised of net loss and other comprehensive income (loss). The Company has no components of other comprehensive income (loss). Therefore net loss equals comprehensive loss for all periods presented and, accordingly, the Consolidated Statements of Comprehensive 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.
	The Company’s accounts receivable are derived from Elan located in Ireland for all periods presented. All of its long-lived assets were held in the United States. Revenue recorded in the Statements of Operations consists of fees earned from the provision of nonclinical research support to Elan, primarily in the areas of safety, toxicology and regulatory. The fees charged to Elan were calculated based on the expenses incurred by the Company in the provision of those R&D services, plus a contractually determined mark-up of those expenses.
	The Company utilizes a third party manufacturer in Switzerland for its clinical drug product supply for therapeutic antibody programs. An inability to obtain drug product supply could have a material adverse impact on the Company's business, financial condition and results of operations.
	Recent Accounting Pronouncements
	As an Emerging Growth Company under the Jumpstart Our Business Startups Act (“JOBS Act”), the Company is eligible to take advantage of certain exemptions from various reporting requirements that apply to other public companies that are not Emerging Growth Companies. The Company has an extended transition period for adopting new or revised accounting standards that have different effective dates for public and private companies until such time as those standards apply to private companies.
	In July 2013, the FASB issued ASU 2013-11, Income Taxes (Topic 740): Presentation of an Unrecognized Tax Benefit when a Net Operating Loss Caryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists, on the financial statement presentation of unrecognized tax benefits. The new guidance provides that a liability related to an unrecognized tax benefit would be presented as a reduction of a deferred tax asset for a net operating loss carryforward, a similar tax loss or a tax credit carryforward if such settlement is required or expected in the event the uncertain tax position is disallowed. The new guidance becomes effective for the Company on January 1, 2015 and will be applied prospectively to unrecognized tax benefits that exist at the effective date with retrospective applications permitted. The Company has presented in these Consolidated Financial Statements its unrecognized tax benefits as a reduction in its deferred tax assets as of December 31, 2013.

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2013
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 $153.3 million and $103.5 million in money market funds included in cash and cash equivalents at December 31, 2013 and 2012, respectively.

Composition_of_Certain_Balance

Composition of Certain Balance Sheet Items	12 Months Ended
	Dec. 31, 2013
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):
		31-Dec
		2013			2012
	Machinery and equipment	$	5,649		$	5,449
	Leasehold improvements	1,927			1,651
	Purchased computer software	85			85
		7,661			7,185
	Less: accumulated depreciation and amortization	(4,289		)	(3,743		)
	Property and equipment, net	$	3,372		$	3,442
	Depreciation expense was $0.7 million, $0.5 million and $0.4 million for the years ended December 31, 2013, 2012, and 2011, respectively.
	Other Current Liabilities
	Other current liabilities consisted of the following (in thousands):
		31-Dec
		2013			2012
	Payroll and related expenses	$	2,800		$	1,592
	Professional services	616			27
	Accrued offering costs	82			—
	Deferred rent	138			51
	Other	254			—
	Other current liabilities	$	3,890		$	1,670

Net_Loss_Per_Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2013
Earnings Per Share [Abstract]	'
Net Loss Per Share	'
	Net Loss Per Share
	Basic net loss per share is calculated by dividing net loss by the weighted-average number of ordinary shares outstanding during the period. Shares used in diluted net loss per share would include the dilutive effect of ordinary shares potentially issuable upon the exercise of stock options outstanding and restricted stock units. However, potentially issuable ordinary shares are not used in computing diluted net loss per share as their effect would be anti-dilutive due to the loss recorded during the periods presented, and therefore diluted net loss per share is equal to basic net loss per share. Prior to the Separation and Distribution, the Company operated as part of Elan and not as a separate entity. As a result, the Company did not have any ordinary shares outstanding prior to December 21, 2012. The calculation of basic and diluted net loss per share assumes that the 14,496,929 ordinary shares issued to Elan shareholders in connection with the separation from Elan were outstanding for the years ended December 31, 2012 and 2011 and that the 3,182,253 ordinary shares issued to Elan upon separation have been outstanding since December 20, 2012.
	Net loss per share was determined as follows (in thousands, except per share amounts):
			Year Ended
			December 31,
			2013			2012			2011
	Numerator:
	Net loss		$	(40,996	)	$	(41,411	)	$	(29,670	)
	Denominator:
	Weighted-average ordinary shares outstanding		18,615			14,593			14,497
	Basic and diluted net loss per share		$	(2.20	)	$	(2.84	)	$	(2.05	)
	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,
		2013			2012		2011
	Stock options to purchase ordinary shares	1,974			1,005		824
	Restricted stock units	—			—		292
	Total	1,974			1,005		1,116

Commitment_and_Contingencies

Commitment and Contingencies	12 Months Ended
	Dec. 31, 2013
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 United States that expires in November 2020 with an estimated annual rent payment of approximately $1.9 million. The lease, as amended, provides for approximately 50,400 of rentable square feet at a base rent that increases annually.
	On November 30, 2013, the Company entered into an amendment of its existing lease agreement relating to the Company’s offices in South San Francisco, California and obtained approximately 13,959 square feet of additional space adjacent to its then existing premises. Under the terms of this amendment, the Company can take occupancy of or sublease the additional space immediately, but is not obligated to begin paying rent or related operating expenses for the additional space until April 1, 2015, subject to certain adjustments if the additional space is sublet prior to that date. Rent for the additional space is at the same rate per rentable square foot as its existing premises and includes certain additional tenancy improvement allowances reimbursable by the landlord. The Company expects to incur an additional $4.6 million in rent and lease-related operating expenses over the seven-year lease term, which expires on November 30, 2020.
	Future minimum payments under operating leases as of December 31, 2013, are as follows (in thousands):
	Year Ended December 31,		Operating Lease
	2014		$	1,302
	2015		1,756
	2016		1,930
	2017		2,009
	2018		2,089
	Thereafter		4,230
	Total future minimum lease payments		$	13,316
	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. Rent expense was $1.3 million, $1.3 million and $1.5 million for the years ended December 31, 2013, 2012, and 2011, 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 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, 2013 and 2012.
	Commitments
	As of December 31, 2013, the Company had non-cancelable purchase commitments to suppliers for $2.7 million of which $758,000 is included in accrued current liabilities, and contractual obligations under license agreements of $1.1 million. The following is a summary of the Company's non-cancelable purchase commitments and contractual obligations as of December 31, 2013 (in thousands):
		Total			2014		2015		2016		2017		2018		Thereafter
	Purchase Obligations	$	2,723		$	2,723	$	—	$	—	$	—	$	—	$	—
	Contractual obligations under license agreements	1,070			85		85		85		85		85		645
	Total	$	3,793		$	2,808	$	85	$	85	$	85	$	85	$	645

Significant_Agreements

Significant Agreements	12 Months Ended
	Dec. 31, 2013
Related Party Transactions [Abstract]	'
Significant Agreements	'
	Significant Agreements
	License, Development, and Commercialization Agreement with Roche
	On December 11, 2013, the Company entered into a License, Development, and Commercialization Agreement, or the License Agreement, with F. Hoffmann-La Roche Ltd and Hoffmann-La Roche Inc., or collectively, Roche, to develop and commercialize certain antibodies that target alpha-synuclein, including PRX002, which are referred to collectively as “Licensed Products.” The effectiveness of the License Agreement is subject to the completion of the customary regulatory clearances, including the expiration of the applicable Hart-Scott-Rodino ("HSR") waiting period. Upon the effectiveness of the License Agreement, the Company will grant to Roche an exclusive, worldwide license to develop, make, have made, use, sell, offer to sell, import, and export the Licensed Products. The Company will retain certain rights to conduct development of the Licensed Products and an option to co-promote PRX002. During the term of the License Agreement, the Company and Roche will work exclusively with each other to research and develop antibody products targeting alpha-synuclein potentially including incorporation of Roche’s proprietary Brain Shuttle™ technology to increase delivery of therapeutic antibodies to the brain. The License Agreement provides that Roche will make an upfront payment to the Company of $30.0 million and a near-term clinical milestone payment of $15.0 million. 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 under the License Agreement. In the United States, 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.
	After the Company files an investigational new drug application with the U.S. Food and Drug Administration for PRX002, 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 United States in the Parkinson’s disease indication. If the Company exercises such option, it may also elect to co-promote additional Licensed Products in the United States approved for Parkinson’s disease. Outside the United States, Roche will have responsibility for developing and commercializing the Licensed Products. Roche bears all costs for product clinical development in support of regulatory approval for all territories 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.
	See Note 13 for further discussion.

Shareholders_Equity

Shareholder's Equity	12 Months Ended
	Dec. 31, 2013
Equity [Abstract]	'
Shareholder's Equity	'
	Shareholders' Equity
	Ordinary Shares
	As of December 31, 2013, the Company had 100,000,000 ordinary shares authorized for issuance with a par value of $0.01 per share and 21,856,261 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, 2013, the Company had 10,000 Euro Deferred Shares authorized for issuance with a nominal value of €22 per share, 1,750 Euro Deferred Shares were issued and no shares are outstanding at December 31, 2013 as the issued Euro Deferred Shares were redeemed upon the separation on December 20, 2012. 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.
	Issuance of Ordinary Shares
	On December 20, 2012, in connection with the Separation and Distribution, the Company issued 14,496,929 ordinary shares to holders of record of Elan ordinary shares and Elan American Depository Shares. Concurrently, the Company issued 3,182,253 ordinary shares to Elan for cash consideration of $26.0 million.
	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 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.

ShareBased_Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]	'
Share-Based Compensation	'
	Share-Based Compensation
	Share-based compensation expense recorded in these consolidated financial statements for the two years ended December 31, 2012 was allocated to the Company based on awards from Elan equity plans granted to Elan employees who have, directly or indirectly, provided services to the Company.
	Share-based compensation expense recorded in these Consolidated Financial Statements for the year ended December 31, 2013 was based on awards from the Prothena Corporation plc 2012 Long Term Incentive Plan (“LTIP”) granted to Prothena employees:
	The following table summarizes share-based compensation expense for the periods presented (in thousands):
		Year Ended December 31,
		2013				2012				2011
	Research and development	$	980			$	6,093			$	2,819
	General and administrative	2,148				5				153
	Total direct expense	$	3,128			$	6,098			$	2,972
	General and administrative — allocated	—				1,445				594
		$	3,128			$	7,543			$	3,566
	The following table summarizes share-based compensation expense by type of award (in thousands):
			Year Ended December 31,
			2013				2012				2011
	Restricted stock units		$	—			$	3,477			$	1,708
	Stock options(1)		3,128				2,621				1,264
	Total direct		$	3,128			$	6,098			$	2,972
	Share-based compensation expense-allocated		—				1,445				594
			$	3,128			$	7,543			3,566
	_________________
	(1)	Includes $0.3 million of share-based compensation expense for the year ended December 31, 2013 related to an option granted to a consultant.
	The Prothena Corporation plc 2012 Long Term Incentive Plan
	The LTIP provides for the issuance of ordinary share-based awards, including restricted shares, restricted stock units (“RSUs”), stock options, share appreciation rights and other equity-based awards, to its employees, officers, directors and consultants. Options under the LTIP may be granted for periods up to ten years. All options issued to date have had a ten year life. Under the LTIP, the Company is authorized to issue a total of 2,650,000 ordinary shares. During the year ended December 31, 2013, the Company granted 1,978,000 share options under its LTIP. The Company's options generally vest over four years. As of December 31, 2013, 676,500 ordinary shares remain available for grant and options to purchase 1,973,500 ordinary shares granted from the LTIP were outstanding with a weighted-average exercise price of approximately $7.50 per share.
	Prothena 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 models 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 highly 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 has no historic employee share option exercise data, the simplified method has been used to estimate the expected life of all options. 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 reported 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 2016 related to unvested share-based payment awards at December 31, 2013 is $7.3 million. The weighted-average period over which the unearned share-based compensation is expected to be recognized is 2.6 years. If there are any modifications or cancellations of the underlying unvested securities, the Company may be required to accelerate, increase or cancel 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 year ended December 31, 2013 was based on awards from Prothena’s LTIP granted to Prothena employees.
	The fair value of the options granted to employees during the year ended December 31, 2013 is 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, 2013
	Expected volatility		84.00%
	Risk-free interest rate		1.20%
	Expected dividend yield		—%
	Expected life (in years)		6
	Weighted average grant date fair value		$5.22
	The following table summarizes the Company’s share option activity during the year ended December 31, 2013:
		Options			Weighted				Weighted		Aggregate
					Average				Average		Intrinsic
					Exercise				Remaining		Value
					Price				Contractual		(in thousands)
									Term (years)
	Outstanding at December 31, 2012	—			$	—			0		$	—
	Granted	1,978,000			7.5
	Canceled	(4,500	)		6.41
	Outstanding at December 31, 2013	1,973,500			$	7.5			9.2		$	37,528
	Vested and expected to vest at December 31, 2013	1,822,838			$	7.47			9.2		$	34,728
	Vested at December 31, 2013	—			$	—			0		$	—
	The following table summarizes information about the Company's share options outstanding as of December 31, 2013:
								Options Outstanding							Options Exercisable
	Range of Exercise Prices							Number of Options			Weighted -		Weighted Average Exercise Price		Number of Options	Weighted Average Exercise Price
											Average
											Remaining
											Contractual Life
											(Years)
	$	6.03		$	6.03			454,500			9.08		$	6.03	—	$	—
	6.41			6.41				857,000			9.08		6.41		—	—
	6.65			6.65				50,000			9.21		6.65		—	—
	6.73			6.73				366,000			9.25		6.73		—	—
	8.21			9.75				103,500			9.41		9.62		—	—
	16.42			16.42				50,000			9.56		16.42		—	—
	17.63			17.63				9,000			9.58		17.63		—	—
	20.04			20.04				13,500			9.75		20.04		—	—
20.17			20.17				40,000			9.67		20.17		—	—
24.26			24.26				30,000			9.84		24.26		—	—
$	6.03		$	24.26			1,973,500			9.17		$	7.5	—	$	—
Elan’s Share-based Compensation Awards
Prior to the Separation and Distribution of the Prothena Business on December 20, 2012, the Company’s employees had received share-based compensation awards under Elan’s equity compensation plans and, therefore, the following disclosures pertain to share-based compensation expense that was allocated to the Prothena Business related to Elan’s share-based equity awards. Elan’s equity award program provided for the issuance of stock options, RSUs and other equity awards to its employees, including employees that have directly and indirectly provided service to the Prothena Business. The share-based payment compensation expense recorded in the Consolidated Financial Statements for the two years ended December 31, 2012 and 2011 includes all of the share-based payment expenses directly attributable to the Prothena Business and an allocation of indirect expenses that have been deemed attributable to the Prothena Business. The Company did not recognize any expense after December 20, 2012 in relation to the existing Elan equity-based awards as the Company’s employees are not required to provide service after the Separation and Distribution in order to receive the benefits of the awards. The share-based compensation expense relating to the changes described below is a non-recurring charge that is directly attributable to Elan as part of the Separation and Distribution of the Prothena Business, and therefore was not recorded in the Company’s Consolidated Financial Statements after December 20, 2012.
Share-based Compensation Expense
Share-based compensation expense was measured and recognized based on estimated grant date fair values. These awards include employee stock options and RSUs, and share purchases related to the Employee Equity Purchase Plan (“EEPP”). Share-based compensation cost for stock options and ordinary shares issued under Elan’s EEPP was estimated at the grant date based on each option’s fair value as calculated using an option-pricing model. Share-based compensation expense for RSUs was measured based on the closing fair market value of Elan’s ordinary shares on the date of grant. The value of awards expected to vest was recognized as an expense over the requisite service periods prior to the separation and distribution. Estimating the fair value of share-based awards as of the grant or vest date using an option-pricing model, such as the binomial model, was affected by Elan’s share price as well as assumptions regarding a number of complex variables. These variables included, but were not limited to, the expected share price volatility over the term of the awards, risk-free interest rates, and actual and projected employee exercise behaviors.
Restricted Share Units
Elan granted RSUs to its employees, including employees that have directly and indirectly provided service to the Prothena Business. The RSUs generally vest between one and three years from the grant date and shares are issued to RSU holders as soon as practicable following vesting. The fair value of services received by the Prothena Business in return for the RSUs is measured by reference to the fair value of the underlying shares at grant date. The total fair value expensed over the vesting terms of RSUs that became fully vested was $0.5 million and $1.1 million in 2012 and 2011, respectively.
Share Options
Share options are granted at the price equal to the market value at the date of grant and will expire on a date not later than 10 years after their grant. Options generally vest between one and four years from the grant date.
Equity-settled share-based payments expense recognized in the “Carve-out” Combined Financial Statements are based on the fair value of the awards measured at the date of grant. The graded-vesting attribution method is used for recognizing share-based compensation expense over the requisite service period for each separately vesting tranche of award as though the awards were, in substance, multiple awards.
The fair value of share options is calculated using a binomial option-pricing model, taking into account the relevant terms and conditions. The binomial option-pricing model is used to estimate the fair value of the Company’s share options because it better reflects the possibility of exercise before the end of the options’ life. The binomial option-pricing model also integrates possible variations in model inputs, such as risk-free interest rates and other inputs, which may change over the life of the options. The amount recognized as an expense is adjusted each period to reflect actual and estimated future levels of vesting.
The implied volatility for traded options on Elan’s shares with remaining maturities of at least one year was used to determine the expected volatility assumption required in the binomial model. The risk-free interest rate assumption is based upon observed interest rates appropriate for the term of the stock option awards. The dividend yield assumption is based on the history and expectation of dividend payouts.
As share-based compensation expense recognized in the “Carve-out” Combined 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 historical experience and estimated future turnover.
The fair value of options granted during the years December 31, 2012 and 2011 was estimated using the binomial option-pricing model with the following weighted-average assumptions:
	Year Ended December 31,
	2012				2011
Expected volatility	60.1		%		49.3		%
Risk-free interest rate	0.9		%		1.7		%
Expected dividend yield	—		%		—		%
Expected life (1)	4.9 - 6.8				4.8 - 7.4
Weighted average fair value	$	6.66			$	2.99
(1)	The expected life of options granted, as derived from the output of the binomial model, ranged from 4.9 to 6.8 years in 2012 and 4.8 to 7.4 years in 2011. The contractual life of the options, which is not more than 10 years from the date of grant, was used as an input into the binomial model.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2013
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	Income Taxes
	Subsequent to the separation from Elan, the Company began to file 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. Prior to the separation from Elan, income taxes as presented in the Consolidated Financial Statements represented current and deferred income taxes of Elan attributed to the Company in a manner that is systematic, rational and consistent with the asset and liability method prescribed by the accounting guidance for income taxes. The Company's income tax provision prior to the separation from Elan was prepared under the “separate return method.” The separate return method applies the accounting guidance for income taxes to the standalone financial statements as if Prothena were a separate taxpayer and a standalone enterprise.
	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,
			2013			2012			2011
	Ireland		$	(42,523	)	$	(35,898	)	$	(27,620	)
	United States		1,942			(5,507		)	(1,624		)
	Loss before provision for income taxes		$	(40,581	)	$	(41,405	)	$	(29,244	)
	Components of the provision for income taxes for each of the fiscal periods presented consisted of the following (in thousands):
			Year Ended December 31,
			2013			2012			2011
	Current:
	U.S. Federal		$	958		$	26		$	426
	U.S. State		(5		)	—			—
	Ireland		—			—			—
	Total current provision		$	953		$	26		$	426
	Deferred:
	U.S. Federal		$	(538	)	$	(20	)	$	—
	U.S. State		—			—			—
	Ireland		—			—			—
	Total deferred provision (benefit)		(538		)	(20		)	$	—
	Total provision for income taxes		$	415		$	6		$	426
	The provision for income taxes differs from the statutory tax rate 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,
			2013			2012			2011
	Taxes at the Irish statutory tax rate of 12.5%		$	(5,073	)	$	(5,176	)	$	(3,656	)
	Income at rates other than the Irish statutory rate		(3,169		)	4			613
	Change in valuation allowance		10,365			5,176			3,656
	Share-based payments		164			—			205
	Tax credits		(1,921		)	—			(392		)
	Other		49			2			—
	Provision for income taxes		$	415		$	6		$	426
	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, 2013 and 2012 are as follows (in thousands):
			31-Dec
			2013			2012
	Deferred tax assets:
	Net operating loss carry forwards		$	15,457		$	8,917
	Tax credits		1,310			—
	Accruals		1,149			79
	Share-based compensation		779			—
	Gross deferred tax assets		18,695			8,996
	Valuation allowance		(17,798		)	(8,917		)
	Net deferred tax assets		897			79
	Deferred tax liability		—			(6		)
	Net deferred tax assets		$	897		$	73
	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 $17.8 million against its deferred tax assets as of December 31, 2013 in relation to deferred tax assets arising from tax credits and net operating losses. The net increase of $8.9 million in the valuation allowance during the year ended December 31, 2013 was primarily due to current year net operating losses and tax credits.
	As of December 31, 2013, certain of the Company's Irish subsidiaries had net operating loss carryovers of $97.2 million which can be carried forward indefinitely but are limited to the same trade/trades. In addition, as of December 31, 2013, the Company had state net operating loss carryforwards of approximately $37.5 million available to reduce future taxable income for the Company's U.S. subsidiary, if any. If not utilized, the state net operating loss carryforward begins expiring in 2032.
	The Company also has federal research and development credits carryforwards of $0.8 million and California research and development credit carryforwards of $0.5 million, respectively, at December 31, 2013. The federal research and development credit carryforwards will expire starting in 2033 if not utilized. The California tax credits can be carried forward indefinitely.
	Cumulative unremitted earnings of the Company’s U.S. subsidiary total approximately $1.5 million at December 31, 2013 The majority of the Company's U.S. subsidiary's cash balance at December 31, 2013 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 Irish taxes (potentially at a rate of 12.5%) if they were to be distributed as dividends. However, as of December 31, 2013 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):
	Gross Unrecognized Tax Benefits at December 31, 2012	$	—
	Additions for tax positions taken in a prior year	—
	Additions for tax positions taken in the current year	480
	Reductions for tax positions taken in the prior year due to settlement	—
	Reductions for tax positions taken in the prior year due to statutes lapsing	—
	Gross Unrecognized Tax Benefits at December 31, 2013	$	480
	If recognized, none of our unrecognized tax benefits as of December 31, 2013 would reduce our annual effective tax rate but would result in a corresponding adjustment to our deferred tax valuation allowance. As of December 31, 2013, 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 major taxing jurisdictions for the Company are Ireland and the United States. The tax years 2012 to 2013 remain subject to examination by the U.S taxing authorities and the tax years 2008 to 2013 remain subject to examination by the Irish taxing authorities.

Employee_Retirement_Plan

Employee Retirement Plan	12 Months Ended
	Dec. 31, 2013
Compensation and Retirement Disclosure [Abstract]	'
Employee Retirement Plan	'
	Employee Retirement Plan
	Prothena 401K Retirement Plan
	In December 2012 (effective January 1, 2013), the Company established 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 a discretionary matching and/or profit sharing contribution as determined solely by the Company. The Company did not record any expense in the year ended December 31, 2012 as no contributions, matching or profit sharing contributions were made under the 401(k) plan. The Company recorded total expense for matching contributions of $0.3 million for the year ended December 31, 2013.
	Elan Pharmaceuticals 401(k) Retirement Savings Plan
	Elan maintains a 401(k) retirement savings plan for its employees based in the United States, including employees that directly and indirectly provided service to the Prothena Business prior to the Separation and Distribution. The Prothena Business recorded total expense for matching contributions of $0.1 million and $0.1 million for the years ended December 31, 2012 and 2011, respectively.

Related_Parties

Related Parties	12 Months Ended
	Dec. 31, 2013
Related Party Transactions [Abstract]	'
Related Parties	'
	Related Parties
	Prior to December 21, 2012, the Prothena Business 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 an indirect wholly owned subsidiary of Perrigo on December 31, 2013.
	On January 17, 2014 the Company filed a registration statement on Form S-3 with the Securities and Exchange Commission (Registration No. 333-193416). This registration statement relates to the resale by the selling shareholder of up to 3,182,253 of the Company's ordinary shares, which represented approximately 14.6% of the ordinary shares issued and outstanding prior to the original date of filing of the registration statement (see Note 13).
	As described elsewhere in these consolidated financial statements, the results of operations of the Prothena business for the time period prior to the separation include transactions with Elan. All of the revenue recognized by the Company for the years ended December 31, 2013, 2012 and 2011 consisted of fees arising from R&D services provided to Elan. Additionally, the results of operations for the time period prior to the separation include certain costs allocated from Elan to the Company for centralized support services.
	The Company has 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 would provide to the Company, and the Company would provide 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, 2013, and 2012, respectively were $0.5 million and $nil, respectively, of which $0.1 million was included in R&D expenses and $0.4 million was included in G&A expenses for year ended December 31, 2013. 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 will provide certain R&D services to Elan. The RDSA has a term of two years. Either party is 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.
	The services provided for under the RDSA include 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 are substantially similar to research services performed by the Company for Elan prior to the separation and distribution.
	The payment terms of the RDSA provide that Elan will 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. There is also a fixed monthly charge of $7,500 to account for lab space and capital equipment used by Elan, for so long as Elan uses such lab space and capital equipment.

Subsequent_Events

Subsequent Events	12 Months Ended
	Dec. 31, 2013
Subsequent Events [Abstract]	'
Subsequent Events	'
	Subsequent Events
	Collaboration with Roche
	On January 22, 2014 the License Agreement with Roche, which is discussed in more detail in Note 7, became effective following the expiration of the HSR waiting period. Consequently, in consideration for the rights granted to Roche under the License Agreement, a one-time, non-refundable, non-creditable payment of $30.0 million was received from Roche in February 2014.
	The Company is currently in the process of evaluating the accounting treatment for this transaction. Based on its preliminary assessment, the Company has identified the deliverables at the inception of the License Agreement as the license, clinical product supply, supply services, development activities and research activities. Through the Company's analysis, it will assess whether each of the deliverables identified has stand alone value, and how the arrangement consideration should be allocated to the deliverables based on the relative selling price methodology. 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 intends to 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 payments will be evaluated separately as the related contingencies are resolved.
	February 2014 Offering
	In February 2014 Elan Science One Limited, or ESOL, an indirect wholly owned subsidiary of Perrigo Company plc, or 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 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 between the Company, Elan and ESOL.

Quarterly_Financial_Informatio

Quarterly Financial Information (Unaudited)	12 Months Ended
	Dec. 31, 2013
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, 2013. 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, 2013
	Revenues	$	171		$	167		$	171		$	167
	Operating expenses	$	9,138		$	11,359		$	9,737		$	10,869
	Net loss	$	(8,951	)	$	(11,302	)	$	(9,689	)	$	(11,054	)
	Net loss per share - basic and diluted	$	(0.51	)	$	(0.64	)	$	(0.55	)	$	(0.52	)
	Year Ended December 31, 2012
	Revenues	$	404		$	735		$	944		$	575
	Operating expenses	$	11,215		$	10,446		$	9,612		$	12,795
	Net loss	$	(10,811	)	$	(9,711	)	$	(8,668	)	$	(12,221	)
	Net loss per share - basic and diluted	$	(0.75	)	$	(0.67	)	$	(0.60	)	$	(0.82	)

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2013
Accounting Policies [Abstract]	'
Basis of Preparation and Presentation of Financial Information	'
	Basis of Preparation and Presentation of Financial Information
	The Prothena Business historically operated as part of Elan and not as a separate stand-alone entity. Prior to the separation on December 20, 2012, the consolidated financial statements of Prothena have been prepared on a “carve-out” basis from the consolidated financial statements of Elan to represent the financial position and performance of Prothena as if the Company had existed on a stand-alone basis and as if Financial Accounting Standards Board (“FASB”) Accounting Standard Codification (“ASC”) Topic 810, “Consolidation” (“ASC 810”) had been applied throughout. The accompanying Consolidated Financial Statements prior to December 21, 2012 include only those assets and liabilities that management has determined are specifically identifiable to Prothena and allocations of direct costs and indirect costs attributable to the Company's operations. The indirect costs included in the Company's Consolidated Financial Statements relate to certain centralized support functions that were provided by Elan. All intragroup transactions within the Prothena Business have been eliminated in the Consolidated Financial Statements and are not disclosed.
	These Consolidated Financial Statements have been prepared in accordance with the accounting principles generally accepted in the United States of America (“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. The financial information for all periods prior to the Separation and Distribution were prepared by aggregating financial information from the components of Prothena as described above. All financial information presented after December 20, 2012 includes the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
	Prior to December 21, 2012, the centralized support functions provided to the Company by Elan included, but were not limited to, accounting, information technology, taxation, legal, corporate strategy, investor relations, corporate governance and other professional services, employee benefit administration, including equity award and pension services, and cash and treasury management. Centralized support costs allocated to the Prothena business for the years ended December 31, 2012 and 2011 were $7.7 million and $4.0 million, respectively. These costs were allocated to the Company for the purposes of preparing the Consolidated Financial Statements based on estimated usage of the resources by the Prothena Business. The estimated usage of the central support resources allocated to the Prothena Business was determined by estimating its portion of the most appropriate driver for each category of central support costs such as headcount or labor hours, depending on the nature of the costs. The Company believes that such allocations were made on a reasonable basis, but may not necessarily be indicative of the costs that would have been incurred if the Prothena Business had operated on a standalone basis.
	Elan used a centralized approach to manage substantially all of its liquid resources and to finance its operations and, as a result, no separate cash accounts for Prothena were historically maintained, and debt and liquid resources maintained at the Elan group level are not included in the accompanying Consolidated Financial Statements prior to the separation. Elan historically funded all of Prothena's operating and capital resource requirements. The parent company equity balance in the Consolidated Financial Statements constitutes Elan's investment in Prothena and represents the excess of total liabilities over total assets (or excess of total assets over total liabilities), including the netting of intercompany funding balances between Prothena and Elan. Changes in parent company equity represent Elan's net investment in Prothena, after giving effect to its net loss, contributions from Elan in the form of share-based compensation to Prothena's employees and net funding provided by Elan.
	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. 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, 2013, 2012, and 2011.
Revenue	'
	Revenue
	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. Advance payments received in excess of amounts earned are classified as deferred revenue until earned. Up-front fees are deferred and amortized to the income statement over the performance period. The performance period is the period over which the Company expects to provide services as determined by the contract provisions.
	The Company recognizes revenue from the delivery of research and development services. Revenue is recognized when all of the following criteria are met: persuasive evidence of an arrangement exists, delivery has occurred or services have been rendered and the contractually specified acceptance criteria have been met, the fee is fixed or determinable, and collectibility is reasonably assured. If sales arrangements contain multiple elements, the Company evaluates whether the components of each arrangement represent separate units of accounting.
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 our Consolidated Financial Statements to the actual services received and efforts expended. As such, expense accruals related to clinical trials are recognized based on our 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 up-front 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).
	Total share-based compensation expense recorded in the Consolidated Financial Statements for the years ended December 31, 2012 and 2011 was allocated to the Company based on awards from Elan equity plans granted to Elan employees who have, directly or indirectly, provided services to the Company.
	With respect to Elan options and RSUs held by Elan employees that became employees of Prothena effective upon the Separation and Distribution:
	•	unvested Elan options and RSUs that would otherwise have vested within twelve months following the effective date of the Separation and Distribution vested immediately upon the Separation and Distribution, with the RSUs (which by their terms are settled upon vesting) settled in Elan ordinary shares or Elan ADSs in accordance with their terms;
	•	other unvested Elan options and RSUs were forfeited; and
	•	all vested Elan options (including options the vesting of which were accelerated as described above) will be required to be exercised for Elan ordinary shares or Elan ADSs within twelve months of the effective date of the Separation and Distribution, or will be forfeited.
	However, for Elan employees who are aged 55 or over with at least five years of service and who became employees of the Company, unvested Elan options and RSUs became fully vested and exercisable upon the Separation and Distribution, with the RSUs (which, by their terms, are settled upon vesting) settled in Elan ordinary shares or Elan ADSs in accordance with their terms, and with Elan options being exercisable for one year following the Separation and Distribution. Similarly, unvested Elan options and RSUs held by Dr. Schenk, became fully vested and exercisable upon the Separation and Distribution, with the RSUs (which, by their terms are settled upon vesting) settled in Elan ordinary shares or Elan ADSs in accordance with their terms, and with Elan options being exercisable for two years following the Separation and Distribution.
	The Company did not recognize any expense after December 20, 2012 in relation to the existing Elan equity-based awards as the Company’s employees are not required to provide service after the Separation and Distribution in order to receive the benefits of the awards. The share-based compensation expense relating to the changes described above is a non-recurring charge that is directly attributable to Elan as part of the Separation and Distribution of the Prothena Business, therefore it was not recorded in the Company’s Consolidated Financial Statements after December 20, 2012.
Income Taxes	'
	Income Taxes
	Subsequent to the separation from Elan, Prothena began to file 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. Prior to the separation from Elan, income taxes as presented in the Consolidated Financial Statements represented current and deferred income taxes of Elan attributed to the Company in a manner that is systematic, rational and consistent with the asset and liability method prescribed by the accounting guidance for income taxes. The Company's income tax provision prior to the separation from Elan was prepared under the “separate return method.” The separate return method applies the accounting guidance for income taxes to the standalone Consolidated Financial Statements as if the Company was a separate taxpayer and a standalone enterprise.
	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.
	Significant 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.
Comprehensive Loss	'
	Comprehensive Loss
	Comprehensive loss is comprised of net loss and other comprehensive income (loss). The Company has no components of other comprehensive income (loss). Therefore net loss equals comprehensive loss for all periods presented and, accordingly, the Consolidated Statements of Comprehensive 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.
	The Company’s accounts receivable are derived from Elan located in Ireland for all periods presented. All of its long-lived assets were held in the United States. Revenue recorded in the Statements of Operations consists of fees earned from the provision of nonclinical research support to Elan, primarily in the areas of safety, toxicology and regulatory. The fees charged to Elan were calculated based on the expenses incurred by the Company in the provision of those R&D services, plus a contractually determined mark-up of those expenses.
Recent Accounting Pronouncements	'
	Recent Accounting Pronouncements
	As an Emerging Growth Company under the Jumpstart Our Business Startups Act (“JOBS Act”), the Company is eligible to take advantage of certain exemptions from various reporting requirements that apply to other public companies that are not Emerging Growth Companies. The Company has an extended transition period for adopting new or revised accounting standards that have different effective dates for public and private companies until such time as those standards apply to private companies.
	In July 2013, the FASB issued ASU 2013-11, Income Taxes (Topic 740): Presentation of an Unrecognized Tax Benefit when a Net Operating Loss Caryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists, on the financial statement presentation of unrecognized tax benefits. The new guidance provides that a liability related to an unrecognized tax benefit would be presented as a reduction of a deferred tax asset for a net operating loss carryforward, a similar tax loss or a tax credit carryforward if such settlement is required or expected in the event the uncertain tax position is disallowed. The new guidance becomes effective for the Company on January 1, 2015 and will be applied prospectively to unrecognized tax benefits that exist at the effective date with retrospective applications permitted. The Company has presented in these Consolidated Financial Statements its unrecognized tax benefits as a reduction in its deferred tax assets as of December 31, 2013.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2013
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
	Property and equipment, net consisted of the following (in thousands):
		31-Dec
		2013			2012
	Machinery and equipment	$	5,649		$	5,449
	Leasehold improvements	1,927			1,651
	Purchased computer software	85			85
		7,661			7,185
	Less: accumulated depreciation and amortization	(4,289		)	(3,743		)
	Property and equipment, net	$	3,372		$	3,442

Composition_of_Certain_Balance1

Composition of Certain Balance Sheet Items (Tables)	12 Months Ended
	Dec. 31, 2013
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
	Property and equipment, net consisted of the following (in thousands):
		31-Dec
		2013			2012
	Machinery and equipment	$	5,649		$	5,449
	Leasehold improvements	1,927			1,651
	Purchased computer software	85			85
		7,661			7,185
	Less: accumulated depreciation and amortization	(4,289		)	(3,743		)
	Property and equipment, net	$	3,372		$	3,442
Schedule of Other Current Liabilities	'
	Other Current Liabilities
	Other current liabilities consisted of the following (in thousands):
		31-Dec
		2013			2012
	Payroll and related expenses	$	2,800		$	1,592
	Professional services	616			27
	Accrued offering costs	82			—
	Deferred rent	138			51
	Other	254			—
	Other current liabilities	$	3,890		$	1,670

Net_Loss_Per_Share_Tables

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2013
Earnings Per Share [Abstract]	'
Calculation of Basic and Diluted Net Income or Loss Per Share	'
	Net loss per share was determined as follows (in thousands, except per share amounts):
			Year Ended
			December 31,
			2013			2012			2011
	Numerator:
	Net loss		$	(40,996	)	$	(41,411	)	$	(29,670	)
	Denominator:
	Weighted-average ordinary shares outstanding		18,615			14,593			14,497
	Basic and diluted net loss per share		$	(2.20	)	$	(2.84	)	$	(2.05	)
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,
		2013			2012		2011
	Stock options to purchase ordinary shares	1,974			1,005		824
	Restricted stock units	—			—		292
	Total	1,974			1,005		1,116

Commitment_and_Contingencies_T

Commitment and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2013
Commitments and Contingencies Disclosure [Abstract]	'
Schedule of Operating Leased Commitment	'
	Future minimum payments under operating leases as of December 31, 2013, are as follows (in thousands):
	Year Ended December 31,		Operating Lease
	2014		$	1,302
	2015		1,756
	2016		1,930
	2017		2,009
	2018		2,089
	Thereafter		4,230
	Total future minimum lease payments		$	13,316
Schedule of Contractual Obligations by Fiscal Year Maturity	'
	he following is a summary of the Company's non-cancelable purchase commitments and contractual obligations as of December 31, 2013 (in thousands):
		Total			2014		2015		2016		2017		2018		Thereafter
	Purchase Obligations	$	2,723		$	2,723	$	—	$	—	$	—	$	—	$	—
	Contractual obligations under license agreements	1,070			85		85		85		85		85		645
	Total	$	3,793		$	2,808	$	85	$	85	$	85	$	85	$	645

ShareBased_Compensation_Tables

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2013
Summary of Share-Based Compensation Expense	'
	The following table summarizes share-based compensation expense for the periods presented (in thousands):
		Year Ended December 31,
		2013				2012				2011
	Research and development	$	980			$	6,093			$	2,819
	General and administrative	2,148				5				153
	Total direct expense	$	3,128			$	6,098			$	2,972
	General and administrative — allocated	—				1,445				594
		$	3,128			$	7,543			$	3,566
	The following table summarizes share-based compensation expense by type of award (in thousands):
			Year Ended December 31,
			2013				2012				2011
	Restricted stock units		$	—			$	3,477			$	1,708
	Stock options(1)		3,128				2,621				1,264
	Total direct		$	3,128			$	6,098			$	2,972
	Share-based compensation expense-allocated		—				1,445				594
			$	3,128			$	7,543			3,566
	_________________
	(1)	Includes $0.3 million of share-based compensation expense for the year ended December 31, 2013 related to an option granted to a consultant.
2012 Long Term Incentive Plan [Member]	'
Fair Value of Options Granted	'
	The fair value of the options granted to employees during the year ended December 31, 2013 is 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, 2013
	Expected volatility		84.00%
	Risk-free interest rate		1.20%
	Expected dividend yield		—%
	Expected life (in years)		6
	Weighted average grant date fair value		$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, 2013:
		Options			Weighted				Weighted		Aggregate
					Average				Average		Intrinsic
					Exercise				Remaining		Value
					Price				Contractual		(in thousands)
									Term (years)
	Outstanding at December 31, 2012	—			$	—			0		$	—
	Granted	1,978,000			7.5
	Canceled	(4,500	)		6.41
	Outstanding at December 31, 2013	1,973,500			$	7.5			9.2		$	37,528
	Vested and expected to vest at December 31, 2013	1,822,838			$	7.47			9.2		$	34,728
	Vested at December 31, 2013	—			$	—			0		$	—
Summary of stock activity by exercise price	'
	The following table summarizes information about the Company's share options outstanding as of December 31, 2013:
								Options Outstanding							Options Exercisable
	Range of Exercise Prices							Number of Options			Weighted -		Weighted Average Exercise Price		Number of Options	Weighted Average Exercise Price
											Average
											Remaining
											Contractual Life
											(Years)
	$	6.03		$	6.03			454,500			9.08		$	6.03	—	$	—
	6.41			6.41				857,000			9.08		6.41		—	—
	6.65			6.65				50,000			9.21		6.65		—	—
	6.73			6.73				366,000			9.25		6.73		—	—
	8.21			9.75				103,500			9.41		9.62		—	—
	16.42			16.42				50,000			9.56		16.42		—	—
	17.63			17.63				9,000			9.58		17.63		—	—
	20.04			20.04				13,500			9.75		20.04		—	—
	20.17			20.17				40,000			9.67		20.17		—	—
	24.26			24.26				30,000			9.84		24.26		—	—
	$	6.03		$	24.26			1,973,500			9.17		$	7.5	—	$	—
Elan [Member]	'
Fair Value of Options Granted	'
	The fair value of options granted during the years December 31, 2012 and 2011 was estimated using the binomial option-pricing model with the following weighted-average assumptions:
		Year Ended December 31,
		2012				2011
	Expected volatility	60.1		%		49.3		%
	Risk-free interest rate	0.9		%		1.7		%
	Expected dividend yield	—		%		—		%
	Expected life (1)	4.9 - 6.8				4.8 - 7.4
	Weighted average fair value	$	6.66			$	2.99
	(1)	The expected life of options granted, as derived from the output of the binomial model, ranged from 4.9 to 6.8 years in 2012 and 4.8 to 7.4 years in 2011. The contractual life of the options, which is not more than 10 years from the date of grant, was used as an input into the binomial model.

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2013
Income Tax Disclosure [Abstract]	'
Loss Before Provision for Income Taxes by Region	'
	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,
			2013			2012			2011
	Ireland		$	(42,523	)	$	(35,898	)	$	(27,620	)
	United States		1,942			(5,507		)	(1,624		)
	Loss before provision for income taxes		$	(40,581	)	$	(41,405	)	$	(29,244	)
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,
			2013			2012			2011
	Current:
	U.S. Federal		$	958		$	26		$	426
	U.S. State		(5		)	—			—
	Ireland		—			—			—
	Total current provision		$	953		$	26		$	426
	Deferred:
	U.S. Federal		$	(538	)	$	(20	)	$	—
	U.S. State		—			—			—
	Ireland		—			—			—
	Total deferred provision (benefit)		(538		)	(20		)	$	—
	Total provision for income taxes		$	415		$	6		$	426
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,
			2013			2012			2011
	Taxes at the Irish statutory tax rate of 12.5%		$	(5,073	)	$	(5,176	)	$	(3,656	)
	Income at rates other than the Irish statutory rate		(3,169		)	4			613
	Change in valuation allowance		10,365			5,176			3,656
	Share-based payments		164			—			205
	Tax credits		(1,921		)	—			(392		)
	Other		49			2			—
	Provision for income taxes		$	415		$	6		$	426
Significant Components of Net Deferred Tax Assets	'
	Significant components of the Company’s net deferred tax assets as of December 31, 2013 and 2012 are as follows (in thousands):
			31-Dec
			2013			2012
	Deferred tax assets:
	Net operating loss carry forwards		$	15,457		$	8,917
	Tax credits		1,310			—
	Accruals		1,149			79
	Share-based compensation		779			—
	Gross deferred tax assets		18,695			8,996
	Valuation allowance		(17,798		)	(8,917		)
	Net deferred tax assets		897			79
	Deferred tax liability		—			(6		)
	Net deferred tax assets		$	897		$	73
Summary of Income Tax Contingencies	'
	A reconciliation of the beginning and ending amounts of unrecognized tax benefits is as follows (in thousands):
	Gross Unrecognized Tax Benefits at December 31, 2012	$	—
	Additions for tax positions taken in a prior year	—
	Additions for tax positions taken in the current year	480
	Reductions for tax positions taken in the prior year due to settlement	—
	Reductions for tax positions taken in the prior year due to statutes lapsing	—
	Gross Unrecognized Tax Benefits at December 31, 2013	$	480

Quarterly_Financial_Informatio1

Quarterly Financial Information (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2013
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, 2013
	Revenues	$	171		$	167		$	171		$	167
	Operating expenses	$	9,138		$	11,359		$	9,737		$	10,869
	Net loss	$	(8,951	)	$	(11,302	)	$	(9,689	)	$	(11,054	)
	Net loss per share - basic and diluted	$	(0.51	)	$	(0.64	)	$	(0.55	)	$	(0.52	)
	Year Ended December 31, 2012
	Revenues	$	404		$	735		$	944		$	575
	Operating expenses	$	11,215		$	10,446		$	9,612		$	12,795
	Net loss	$	(10,811	)	$	(9,711	)	$	(8,668	)	$	(12,221	)
	Net loss per share - basic and diluted	$	(0.75	)	$	(0.67	)	$	(0.60	)	$	(0.82	)

Organization_Additional_Inform

Organization - Additional Information (Detail) (USD $)	0 Months Ended	12 Months Ended				1 Months Ended	12 Months Ended		1 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 20, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Oct. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2013
						Underwritten Public And Over-Allotment Offering [Member]	Ordinary Shares	Ordinary Shares	Ordinary Shares
									Underwritten Public And Over-Allotment Offering [Member]
Organization [Line Items]	'	'	'	'	'	'	'	'	'
Net funding provided by Elan	$125,000	'	$142,233	$20,292	'	'	'	'	'
Accumulated deficit	'	41,341	345	'	'	'	'	'	'
Cash and cash equivalents	'	176,677	124,860	0	0	'	'	'	'
Number of ordinary shares sold	14,496,929	'	'	'	'	'	4,177,079	14,496,929	4,177,079
Net proceeds received for the sale of shares	'	$84,453	'	'	'	$84,453	$42	$145	'

Summary_of_Significant_Account3

Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Centralized Costs (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Accounting Policies [Abstract]	'	'
Centralized support costs	$7.70	$4

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2013
Property, Plant and Equipment [Line Items]	'
Purchased computer software, useful life (in years)	'4 years
Minimum [Member]	'
Property, Plant and Equipment [Line Items]	'
Property and equipment, useful life (in years)	'4 years
Maximum [Member]	'
Property, Plant and Equipment [Line Items]	'
Property and equipment, useful life (in years)	'7 years

Summary_of_Significant_Account5

Summary of Significant Accounting Policies - RSU (Details) (Elan [Member])	12 Months Ended
	Dec. 31, 2013
Restricted stock units [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Minimum age required for award	55
Minimum years of service required for award (in years)	'5 years
Exercisable period of options (in years)	'1 year
Dr. Schenk [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Exercisable period of options (in years)	'2 years

Fair_Value_Measurements_Additi

Fair Value Measurements - Additional Information (Detail) (Level 1 [Member], USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Level 1 [Member]	'	'
Fair Value Inputs, Assets, Quantitative Information [Line Items]	'	'
Money Market Funds at Carrying Value	$153.30	$103.50

Composition_of_Certain_Balance2

Composition of Certain Balance Sheet Items - Schedule of Property and Equipment (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Composition of Certain Balance Sheet Items [Abstract]	'	'
Machinery and equipment	$5,649	$5,449
Leasehold improvements	1,927	1,651
Purchased computer software	85	85
Property and equipment, gross	7,661	7,185
Less: accumulated depreciation and amortization	-4,289	-3,743
Property and equipment, net	$3,372	$3,442

Composition_of_Certain_Balance3

Composition of Certain Balance Sheet Items - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Composition of Certain Balance Sheet Items [Abstract]	'	'	'
Depreciation expense	$0.70	$0.50	$0.40

Composition_of_Certain_Balance4

Composition of Certain Balance Sheet Items - Schedule of Other Current Liabilities (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Composition of Certain Balance Sheet Items [Abstract]	'	'
Payroll and related expenses	$2,800	$1,592
Professional services	616	27
Accrued offering costs	82	0
Deferred rent	138	51
Other	254	0
Other current liabilities	$3,890	$1,670

Net_Loss_Per_Share_Additional_

Net Loss Per Share - Additional Information (Detail)	0 Months Ended		12 Months Ended
	Dec. 20, 2012	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
		Elan [Member]	Elan [Member]	Elan [Member]
Earnings Per Share [Line Items]	'	'	'	'
Number of ordinary shares issued, New Issues	14,496,929	'	14,496,929	14,496,929
Issuance of ordinary shares to Elan, shares	'	3,182,253	'	'

Net_Loss_Per_Share_Calculation

Net Loss Per Share - Calculation of Basic and Diluted Net Income or Loss Per Share (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Numerator:	'	'	'	'	'	'	'	'	'	'	'
Net loss	($11,054)	($9,689)	($11,302)	($8,951)	($12,221)	($8,668)	($9,711)	($10,811)	($40,996)	($41,411)	($29,670)
Denominator:	'	'	'	'	'	'	'	'	'	'	'
Weighted-average ordinary shares outstanding (in shares)	'	'	'	'	'	'	'	'	18,615	14,593	14,497
Basic and diluted net loss per share (in dollars per share)	($0.52)	($0.55)	($0.64)	($0.51)	($0.82)	($0.60)	($0.67)	($0.75)	($2.20)	($2.84)	($2.05)

Net_Loss_Per_Share_Ordinary_Sh

Net Loss Per Share - Ordinary Shares Equivalent Not Included in Diluted Net Loss Per Share (Detail)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'
Equivalent ordinary shares not included in diluted net loss per share	1,974	1,005	1,116
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	1,974	1,005	824
Restricted stock units [Member]	'	'	'
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'
Equivalent ordinary shares not included in diluted net loss per share	0	0	292

Commitment_and_Contingencies_L

Commitment and Contingencies - Lease Narrative (Details) (USD $)	0 Months Ended	12 Months Ended
	Nov. 30, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
	sqft	sqft
Long-term Purchase Commitment [Line Items]	'	'	'	'
Expiration of lease	'	30-Nov-20	'	'
Rent expense	'	$1,300,000	$1,300,000	$1,500,000
Purchase Obligation, Due in Next Twelve Months	'	2,723,000	'	'
Area of rentable square feet	13,959	50,400	'	'
Operating lease, term	'7 years	'	'	'
Forecast [Member]	'	'	'	'
Long-term Purchase Commitment [Line Items]	'	'	'	'
Rent expense	4,600,000	$1,900,000	'	'

Commitment_and_Contingencies_C

Commitment and Contingencies - Commitment Narrative (Details) (USD $)	Dec. 31, 2013
In Thousands, unless otherwise specified
Long-term Purchase Commitment [Line Items]	'
Contractual Obligation	$1,070
Purchase Obligation	2,723
Accrued current liabilities [Member]	'
Long-term Purchase Commitment [Line Items]	'
Commitment to suppliers for purchases	758
License agreements [Member]	'
Long-term Purchase Commitment [Line Items]	'
Contractual Obligation	$1,070

Commitment_and_Contingencies_S

Commitment and Contingencies - Schedule of Operating Lease Commitment (Details) (USD $)	Dec. 31, 2013
In Thousands, unless otherwise specified
Commitments and Contingencies Disclosure [Abstract]	'
2014	$1,302
2015	1,756
2016	1,930
2017	2,009
2018	2,089
Thereafter	4,230
Total future minimum lease payments	$13,316

Commitment_and_Contingencies_C1

Commitment and Contingencies Commitment and Contingencies - Contractual Obligations (Details) (USD $)	Dec. 31, 2013
In Thousands, unless otherwise specified
Purchase Obligation, Fiscal Year Maturity [Abstract]	'
Total	$2,723
2014	2,723
2015	0
2016	0
2017	0
2018	0
Thereafter	0
Contractual Obligation, Fiscal Year Maturity [Abstract]	'
Total	1,070
2014	85
2015	85
2016	85
2017	85
2018	85
Thereafter	645
Purchase and Contractual Obligations, Fiscal Year Maturity [Abstract]	'
Total	3,793
2014	2,808
2015	85
2016	85
2017	85
2018	85
Thereafter	$645

Significant_Agreements_Details

Significant Agreements (Details) (Collaborative Arrangement [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013
License Agreement [Line Items]	'
Portion of revenue and expenses attributable to company, percentage	30.00%
Roche [Member]	'
License Agreement [Line Items]	'
Upfront payment	30
Potential near-term clinical milestone payment	15
Potential payment upon achievement of development, regulatory and various first commercial sales milestones	380
Potential payment for achievement of non U.S.commercial sales milestones	175
Portion of revenue and expenses attributable to company, percentage	70.00%
Potential alternative commercial sales milestone payment	155

Shareholders_Equity_Ordinary_S

Shareholder's Equity (Ordinary Shares/Euro Deferred Shares) (Details)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012
	USD ($)	EUR (€)	USD ($)	EUR (€)
	vote	vote
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)	$0.01	'	$0.01	'
Ordinary shares, number of issued shares	21,856,261	21,856,261	17,679,182	17,679,182
Ordinary shares, number of outstanding shares	21,856,261	21,856,261	17,679,182	17,679,182
Votes per share	1	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)	'	€ 22	'	€ 22
Euro deferred shares, number of issued shares	'	'	1,750	1,750
Euro deferred shares, number of outstanding shares	0	0	0	0

Shareholders_Equity_Issuance_o

Shareholder's Equity (Issuance of Ordinary Shares) (Details) (USD $)	0 Months Ended		12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2012	Dec. 20, 2012	Dec. 31, 2012	Dec. 31, 2011
Class of Stock [Line Items]	'	'	'	'
Number of ordinary shares issued, New Issues	'	14,496,929	'	'
Issuance of ordinary shares to Elan, value	'	'	26,000	'
Elan [Member]	'	'	'	'
Class of Stock [Line Items]	'	'	'	'
Number of ordinary shares issued, New Issues	'	'	14,496,929	14,496,929
Issuance of ordinary shares to Elan, shares	3,182,253	'	'	'
Issuance of ordinary shares to Elan, value	'	$26,000	'	'

Shareholders_Equity_October_20

Shareholder's Equity (October 2013 Offering) (Details) (USD $)	0 Months Ended	12 Months Ended				1 Months Ended
	Dec. 20, 2012	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2013	Oct. 31, 2013	Oct. 31, 2013	Oct. 31, 2013	Oct. 31, 2013
			Ordinary Shares	Ordinary Shares	Underwritten Public Offering [Member]	Underwritten Public Offering [Member]	Underwritten Public And Over-Allotment Offering [Member]	Underwritten Public And Over-Allotment Offering [Member]	Janssen [Member]
						Ordinary Shares		Ordinary Shares	Ordinary Shares
Class of Stock [Line Items]	'	'	'	'	'	'	'	'	'
Ordinary shares sold	'	'	'	'	'	6,796,500	'	'	'
Ordinary shares sold (in dollars per share)	'	'	'	'	$22	'	'	'	'
Number of ordinary shares issued, New Issues	14,496,929	'	4,177,079	14,496,929	'	'	'	4,177,079	'
Shares sold pursuant to underwritten public offering	'	'	'	'	'	'	'	'	2,619,421
Stock Issued During Period, Value, New Issues	'	$84,453,000	$42,000	$145,000	'	'	$84,453,000	'	'
Deferred offering costs	'	$7,400,000	'	'	'	'	'	'	'

ShareBased_Compensation_Summar

Share-Based Compensation - Summary of Share-Based Compensation Expense (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'		'		'
Share-based compensation expense	$3,128,000		$6,098,000		$2,972,000
Total Share-based Compensation Expense	3,128,000		7,543,000		3,566,000
Restricted stock units [Member]	'		'		'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'		'		'
Share-based compensation expense	0		3,477,000		1,708,000
Options to purchase ordinary shares [Member]	'		'		'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'		'		'
Share-based compensation expense	3,128,000	[1]	2,621,000	[1]	1,264,000	[1]
Consultant [Member]	'		'		'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'		'		'
Share-based compensation expense on option	300,000		'		'
Research and development [Member]	'		'		'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'		'		'
Share-based compensation expense	980,000		6,093,000		2,819,000
General and administrative [Member]	'		'		'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'		'		'
Allocated Share-based Compensation Expense	0		1,445,000		594,000
Share-based compensation expense	$2,148,000		$5,000		$153,000
[1]	Includes $0.3 million of share-based compensation expense for the year ended December 31, 2013 related to an option granted to a consultant

ShareBased_Compensation_Fair_V

Share-Based Compensation - Fair Value of Options Granted (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2011
	Elan [Member]	Elan [Member]	2012 Long Term Incentive Plan [Member]	Minimum [Member]	Minimum [Member]	Maximum [Member]	Maximum [Member]
				Elan [Member]	Elan [Member]	Elan [Member]	Elan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'	'	'	'
Expected volatility	60.10%	49.30%	83.99%	'	'	'	'
Risk-free interest rate	0.90%	1.70%	1.21%	'	'	'	'
Expected dividend yield	0.00%	0.00%	0.00%	'	'	'	'
Expected life	'10 years	'10 years	'6 years	'4 years 10 months 24 days	'4 years 9 months 18 days	'6 years 9 months 18 days	'7 years 4 months 24 days
Weighted average fair value	$6.66	$2.99	$5.22	'	'	'	'

Sharebased_Compensation_Shareb

Share-based Compensation - Share-based Compensation Plan - Option Activity (Detail) (2012 Long Term Incentive Plan [Member], USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2013
2012 Long Term Incentive Plan [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]	'
Options, Outstanding beginning balance	0
Options, granted	1,978,000
Options, canceled	-4,500
Options, Outstanding ending balance	1,973,500
Options, Vested and expected to vest ending balance	1,822,838
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]	'
Weighted Average Exercise Price, Outstanding beginning balance	$0
Weighted Average Exercise Price, Granted	$7.50
Weighted Average Exercise Price, Canceled	$6.41
Weighted Average Exercise Price, Outstanding ending balance	$7.50
Weighted Average Exercise Price, Vested and expected to vest ending balance	$7.47
Weighted Average Remaining Contractual Life (Years), Outstanding ending balance	'9 years 2 months 1 day
Weighted Average Remaining Contractual Life (Years), Vested and expected to vest ending balance	'9 years 2 months 1 day
Aggregate Intrinsic Value, Outstanding ending balance	$37,528
Aggregate Intrinsic Value, Vested and expected to vest ending balance	$34,728

ShareBased_Compensation_Exerci

Share-Based Compensation - Exercise Price (Details) (2012 Long Term Incentive Plan [Member], USD $)	12 Months Ended
	Dec. 31, 2013
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)	$24.26
Number of options outstanding	1,973,500
Options outstanding, weighted-average remaining contractual life	'9 years 2 months 1 day
Options outstanding, weighted average exercise price (in dollars per share)	$7.50
$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	454,500
Options outstanding, weighted-average remaining contractual life	'9 years 29 days
Options outstanding, 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	857,000
Options outstanding, weighted-average remaining contractual life	'9 years 29 days
Options outstanding, weighted average exercise price (in dollars per share)	$6.41
$6.65 - $6.65 [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)	$6.65
Number of options outstanding	50,000
Options outstanding, weighted-average remaining contractual life	'9 years 2 months 16 days
Options outstanding, weighted average exercise price (in dollars per share)	$6.65
$6.73 - $6.73 [Member]	'
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]	'
Exercise price, lower limit (in dollars per share)	$6.73
Exercise price, upper limit (in dollars per share)	$6.73
Number of options outstanding	366,000
Options outstanding, weighted-average remaining contractual life	'9 years 3 months
Options outstanding, weighted average exercise price (in dollars per share)	$6.73
$8.21 - $9.75 [Member]	'
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]	'
Exercise price, lower limit (in dollars per share)	$8.21
Exercise price, upper limit (in dollars per share)	$9.75
Number of options outstanding	103,500
Options outstanding, weighted-average remaining contractual life	'9 years 4 months 28 days
Options outstanding, weighted average exercise price (in dollars per share)	$9.62
$16.42 - $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)	$16.42
Exercise price, upper limit (in dollars per share)	$16.42
Number of options outstanding	50,000
Options outstanding, weighted-average remaining contractual life	'9 years 6 months 22 days
Options outstanding, weighted average exercise price (in dollars per share)	$16.42
$17.63 - $17.63 [Member]	'
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]	'
Exercise price, lower limit (in dollars per share)	$17.63
Exercise price, upper limit (in dollars per share)	$17.63
Number of options outstanding	9,000
Options outstanding, weighted-average remaining contractual life	'9 years 6 months 29 days
Options outstanding, weighted average exercise price (in dollars per share)	$17.63
$20.04 - $20.04 [Member]	'
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]	'
Exercise price, lower limit (in dollars per share)	$20.04
Exercise price, upper limit (in dollars per share)	$20.04
Number of options outstanding	13,500
Options outstanding, weighted-average remaining contractual life	'9 years 9 months
Options outstanding, weighted average exercise price (in dollars per share)	$20.04
$20.17 - $20.17 [Member]	'
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]	'
Exercise price, lower limit (in dollars per share)	$20.17
Exercise price, upper limit (in dollars per share)	$20.17
Number of options outstanding	40,000
Options outstanding, weighted-average remaining contractual life	'9 years 8 months 1 day
Options outstanding, weighted average exercise price (in dollars per share)	$20.17
$24.26 - $24..26 [Member]	'
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]	'
Exercise price, lower limit (in dollars per share)	$24.26
Exercise price, upper limit (in dollars per share)	$24.26
Number of options outstanding	30,000
Options outstanding, weighted-average remaining contractual life	'9 years 10 months 2 days
Options outstanding, weighted average exercise price (in dollars per share)	$24.26

ShareBased_Compensation_Additi

Share-Based Compensation - Additional Information (Detail) (USD $)	12 Months Ended		12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2012
	2012 Long Term Incentive Plan [Member]	2012 Long Term Incentive Plan [Member]	Restricted stock units [Member]	Restricted stock units [Member]	Restricted stock units [Member]	Restricted stock units [Member]	Stock Option [Member]	Stock Option [Member]	Stock Option [Member]
			Elan [Member]	Elan [Member]	Minimum [Member]	Maximum [Member]	Elan [Member]	Minimum [Member]	Maximum [Member]
					Elan [Member]	Elan [Member]		Elan [Member]	Elan [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]	'	'	'	'	'	'	'	'	'
Grant period	'10 years	'	'	'	'	'	'10 years	'	'
Authorized shares for issuance	2,650,000	'	'	'	'	'	'	'	'
Number of stock options granted to employees	1,978,000	'	'	'	'	'	'	'	'
Vesting period	'4 years	'	'	'	'1 year	'3 years	'	'1 year	'4 years
Fair value of related costs being expensed	'	'	$0.50	$1.10	'	'	'	'	'
Number of shares available for grant	676,500	'	'	'	'	'	'	'	'
Options outstanding	1,973,500	0	'	'	'	'	'	'	'
Weighted average exercise price	$7.50	$0	'	'	'	'	'	'	'
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options	$7.30	'	'	'	'	'	'	'	'
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	'2 years 7 months 13 days	'	'	'	'	'	'	'	'

Income_Taxes_Loss_Before_Provi

Income Taxes - Loss Before Provision for Income Taxes by Region (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Disclosure [Abstract]	'	'	'
Ireland	($42,523)	($35,898)	($27,620)
United States	1,942	-5,507	-1,624
Loss before provision for income taxes	($40,581)	($41,405)	($29,244)

Income_Taxes_Components_of_the

Income Taxes - Components of the Provision for Income Taxes (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Current:	'	'	'
U.S. Federal	$958	$26	$426
U.S. State	-5	0	0
Ireland	0	0	0
Total current provision	953	26	426
Deferred:	'	'	'
U.S. Federal	-538	-20	0
U.S. State	0	0	0
Ireland	0	0	0
Deferred income taxes	-538	-20	0
Provision for income taxes	$415	$6	$426

Income_Taxes_Reconciliation_Be

Income Taxes - Reconciliation Between Income Taxes Computed at the Standard Irish Statutory Tax Rate and the Provision for Income Taxes (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Disclosure [Abstract]	'	'	'
Irish Tax Rate	12.50%	12.50%	12.50%
Taxes at the Irish statutory tax rate of 12.5%	($5,073)	($5,176)	($3,656)
Income at rates other than the Irish statutory rate	-3,169	4	613
Change in valuation allowance	10,365	5,176	3,656
Share-based payments	164	0	205
Tax credits	-1,921	0	-392
Other	49	2	0
Provision for income taxes	$415	$6	$426

Income_Taxes_Significant_Compo

Income Taxes - Significant Components of Net Deferred Tax Assets (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Deferred tax assets:	'	'
Net operating loss carry forwards	$15,457	$8,917
Tax credits	1,310	0
Accruals	1,149	79
Share-based compensation	779	0
Gross deferred tax assets	18,695	8,996
Valuation allowance	-17,798	-8,917
Net deferred tax assets	897	79
Deferred tax liability	0	-6
Net deferred tax assets	$897	$73

Income_Taxes_Reconciliation_of

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]	'
Gross Unrecognized Tax Benefits at December 31, 2012	$0
Additions for tax positions taken in a prior year	0
Additions for tax positions taken in the current year	480
Reductions for tax positions taken in the prior year due to settlement	0
Reductions for tax positions taken in the prior year due to statutes lapsing	0
Gross Unrecognized Tax Benefits at December 31, 2013	$480

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Taxes [Line Items]	'	'	'
Valuation allowance	($17,798,000)	($8,917,000)	'
Increase in valuation allowance	8,900,000	'	'
Tax credits	1,310,000	0	'
Federal statutory income tax rate	12.50%	12.50%	12.50%
Ireland [Member]	'	'	'
Income Taxes [Line Items]	'	'	'
Operating loss carryforwards	97,200,000	'	'
Federal statutory income tax rate	12.50%	'	'
United States [Member]	'	'	'
Income Taxes [Line Items]	'	'	'
Tax credits	800,000	'	'
Cumulative unremitted earnings	1,500,000	'	'
California [Member]	'	'	'
Income Taxes [Line Items]	'	'	'
Operating loss carryforwards	37,500,000	'	'
Tax credits	$500,000	'	'

Employee_Retirement_Plan_Detai

Employee Retirement Plan (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
		Elan [Member]	Elan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'	'	'
Maximum annual contributions per employee, percent	100.00%	'	'
Employer matching contribution, percent of employees' gross pay	3.00%	'	'
Recorded expense for matching contributions	$0.30	$0.10	$0.10

Related_Parties_Additional_Inf

Related Parties - Additional Information (Detail) (USD $)

1 Months Ended

12 Months Ended

1 Months Ended

0 Months Ended

1 Months Ended

0 Months Ended

12 Months Ended

1 Months Ended

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 20, 2012

Dec. 31, 2012

Jan. 17, 2014

Dec. 31, 2012

Feb. 28, 2014

Transitional Services Agreement [Member]

Transitional Services Agreement [Member]

Transitional Services Agreement [Member]

Transitional Services Agreement [Member]

Transitional Services Agreement [Member]

R&D Services Agreement [Member]

Elan [Member]

Elan [Member]

Elan [Member]

Subsequent Event [Member]

Ordinary Shares

Ordinary Shares

Research and development [Member]

General and administrative [Member]

Transitional Services Agreement [Member]

Perrigo [Member]

Subsequent Event [Member]

Perrigo [Member]

Related Party Transaction [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

Percentage of ownership interest acquired

'

'

'

'

'

'

'

18.00%

'

'

'

'

Shares sold pursuant to underwritten public offering

'

'

'

'

'

'

'

'

'

'

'

3,182,253

Issuance of ordinary shares to Elan, shares

'

'

'

'

'

'

3,182,253

'

'

'

3,182,253

'

Percentage of total shares sold

'

'

'

'

'

'

'

'

'

14.60%

'

'

Additional percentage of employee service payment

40.00%

'

'

'

'

'

'

'

40.00%

'

'

'

TSA expenses recognized

'

$500,000

$0

$100,000

$400,000

'

'

'

'

'

'

'

Research and development service agreement (in years)

'

'

'

'

'

'2 years

'

'

'

'

'

'

Fixed service fee, annual rate

'

'

'

'

'

500,000

'

'

'

'

'

'

Variable service fee, annual rate

'

'

'

'

'

250,000

'

'

'

'

'

'

Fixed service fee per employee, annual rate

'

'

'

'

'

250,000

'

'

'

'

'

'

Percentage of mark-up applied to fixed and variable charge

'

'

'

'

'

10.00%

'

'

'

'

'

'

Fixed monthly charge for lab space and capital equipment used

'

'

'

'

'

$7,500

'

'

'

'

'

'

Subsequent_Event_Details

Subsequent Event (Details) (Subsequent Event [Member], USD $)	1 Months Ended
In Millions, except Share data, unless otherwise specified	Feb. 28, 2014
Perrigo [Member]	'
Subsequent Event [Line Items]	'
Share price (in dollars per share)	$26
Collaborative Arrangement [Member] | Roche [Member]	'
Subsequent Event [Line Items]	'
Upfront payment	$30
Ordinary Shares | Perrigo [Member]	'
Subsequent Event [Line Items]	'
Shares sold pursuant to underwritten public offering	3,182,253

Quarterly_Financial_Informatio2

Quarterly Financial Information (Unaudited) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]	'	'	'	'	'	'	'	'	'	'	'
Revenues	$167	$171	$167	$171	$575	$944	$735	$404	$676	$2,658	$507
Operating expenses	10,869	9,737	11,359	9,138	12,795	9,612	10,446	11,215	41,103	44,068	29,751
Net loss	($11,054)	($9,689)	($11,302)	($8,951)	($12,221)	($8,668)	($9,711)	($10,811)	($40,996)	($41,411)	($29,670)
Basic and diluted net loss per share (in dollars per share)	($0.52)	($0.55)	($0.64)	($0.51)	($0.82)	($0.60)	($0.67)	($0.75)	($2.20)	($2.84)	($2.05)