Prothena (PRTA) 10-Q 6 May 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	Apr. 24, 2015
Document Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	PRTA
Entity Registrant Name	Prothena Corp plc
Entity Central Index Key	1559053
Current Fiscal Year End Date	-19
Entity Filer Category	Accelerated Filer
Entity Ordinary Shares Outstanding		31,285,885

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$279,857	$293,579
Receivable from Roche	1,357	1,729
Receivable from related party	30	30
Deferred tax assets	163	167
Prepaid expenses and other current assets	3,012	3,770
Total current assets	284,419	299,275
Non-current assets:
Property and equipment, net	2,956	3,121
Deferred tax assets, non-current	2,093	1,720
Other assets	117	0
Total non-current assets	5,166	4,841
Total assets	289,585	304,116
Current liabilities:
Accounts payable	1,990	4,722
Accrued research and development	3,662	2,285
Income taxes payable	0	57
Other current liabilities	3,892	4,975
Total current liabilities	9,544	12,039
Non-current liabilities:
Income taxes payable, non-current	98	98
Deferred rent	2,391	2,090
Other liabilities	126	0
Total non-current liabilities	2,615	2,188
Total liabilities	12,159	14,227
Commitments and contingencies (Note 6)
Shareholders’ equity:
Euro deferred shares, €22 nominal value: Authorized shares - 10,000 at March 31, 2015 and December 31, 2014 Issued and outstanding shares - none at March 31, 2015 and December 31, 2014	0	0
Ordinary shares, $0.01 par value: Authorized shares - 100,000,000 at March 31, 2015 and December 31, 2014 Issued and outstanding shares - 27,466,407 March 31, 2015 and 27,388,005 at December 31, 2014	275	274
Additional paid-in capital	340,844	338,106
Accumulated deficit	-63,693	-48,491
Total shareholders’ equity	277,426	289,889
Total liabilities and shareholders’ equity	$289,585	$304,116

Condensed_Consolidated_Balance1

Condensed Consolidated Balance Sheets (Parenthetical)	Mar. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2014
	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	27,466,407	27,466,407	27,388,005	27,388,005
Ordinary shares, number of outstanding shares	27,466,407	27,466,407	27,388,005	27,388,005

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Operations (USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Income Statement [Abstract]
Collaboration revenue	$593	$32,096
Revenue—related party	0	138
Total revenue	593	32,234
Operating expenses:
Research and development	10,573	9,342
General and administrative	5,049	4,873
Total operating expenses	15,622	14,215
Income (loss) from operations	-15,029	18,019
Other income (expense):
Interest income	26	19
Other income (expense), net	67	-35
Total other income (expense)	93	-16
Income (loss) before income taxes	-14,936	18,003
Provision for income taxes	266	151
Net income (loss)	($15,202)	$17,852
Basic net income (loss) per share	($0.55)	$0.82
Diluted net income (loss) per share	($0.55)	$0.78
Shares used to compute basic net income (loss) per share (in shares)	27,401	21,884
Shares used to compute diluted net income (loss) per share (in shares)	27,401	22,942

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Cash Flows (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Operating activities
Net income (loss)	($15,202)	$17,852
Adjustments to reconcile net loss to cash provided by (used in) operating activities:
Depreciation and amortization	193	175
Share-based compensation	1,703	1,343
Excess tax benefit from share-based award exercises	-390	-207
Deferred income taxes	-369	-159
Loss on sublease	261	0
Changes in operating assets and liabilities:
Receivable from Roche	372	-2,161
Receivable from related party	0	12
Other assets	933	-707
Accounts payable, accruals and other liabilities	-2,221	1,746
Net cash provided by (used in) operating activities	-14,720	17,894
Investing activities
Purchases of property and equipment	-28	-26
Net cash used in investing activities	-28	-26
Financing activities
Public offering costs	0	-10
Proceeds from issuance of ordinary shares upon exercise of stock options	636	310
Excess tax benefit from share-based award exercises	390	207
Net cash provided by financing activities	1,026	507
Net increase (decrease) in cash and cash equivalents	-13,722	18,375
Cash and cash equivalents, beginning of the year	293,579	176,677
Cash and cash equivalents, end of the period	279,857	195,052
Supplemental disclosures of cash flow information
Cash paid for income taxes, net of refunds	442	436
Supplemental disclosures of non-cash investing and financing activities
Acquisition of property and equipment under accounts payable and accrued liabilities	0	10
Offering costs in accounts payable and accrued liabilities	117	43
Receivable from stock option exercises	$16	$0

Organization

Organization	3 Months Ended
	Mar. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization	Organization
	Description of Business
	Prothena Corporation plc and its subsidiaries (“Prothena” or the “Company”) is a late-stage clinical biotechnology company focused on the discovery, development and commercialization of novel protein immunotherapy programs for the potential treatment of diseases that involve amyloid or cell adhesion. The Company is developing antibody-based product candidates that target a number of potential indications including AL amyloidosis (NEOD001), Parkinson’s disease and other related synucleinopathies (PRX002) and psoriasis and other inflammatory diseases (PRX003).
	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”), on December 20, 2012. 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.
	Prothena's business consists of a substantial portion of Elan's former drug discovery business platform, including Neotope Biosciences Limited (now named Prothena Biosciences Limited) and its wholly owned subsidiaries Onclave Therapeutics Limited (now named Prothena Therapeutics Limited) and Prothena Biosciences Inc (which for the period prior to the Separation and Distribution are collectively 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.
	Liquidity and Business Risks
	As of March 31, 2015, the Company had an accumulated deficit of $63.7 million and cash and cash equivalents of $279.9 million.
	Based on the Company's business plans, management believes that the Company's cash and cash equivalents at March 31, 2015 are sufficient to meet its obligations for at least the next twelve months. To operate beyond such period, or if the Company elects to increase its spending on development programs significantly above current long-term plans or enters into potential licenses and or other acquisitions of complementary technologies, products or companies, the Company may need additional capital. The Company expects to continue to finance future cash needs that exceed its cash from operating activities primarily through its current cash and cash equivalents, its collaboration with Roche, and to the extent necessary, through proceeds from public or private equity or debt financings, loans and other collaborative agreements with corporate partners or other arrangements.
	The Company is subject to a number of risks, including but not limited to: the uncertainty of the Company's research and development (“R&D”) efforts resulting in future successful commercial products; obtaining regulatory approval for its product candidates; its ability to successfully commercialize its product candidates, if approved; significant competition from larger organizations; reliance on the proprietary technology of others; dependence on key personnel; uncertain patent protection; dependence on corporate partners and collaborators; and possible restrictions on reimbursement from governmental agencies and healthcare organizations, as well as other changes in the healthcare industry.
	The Company is dependent on Boehringer Ingelheim to manufacture clinical supplies for its therapeutic antibody programs. An inability to obtain product supply on a timely basis 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	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies
	Basis of Preparation and Presentation of Financial Information
	The accompanying interim Condensed Consolidated Financial Statements have been prepared in accordance with the accounting principles generally accepted in the U.S. (“GAAP”) and with the instructions for Form 10-Q and Regulations S-X statements. Accordingly, they do not include all of the information and notes required for complete financial statements. These interim Condensed Consolidated Financial Statements should be read in conjunction with the Consolidated Financial Statements and Notes thereto contained in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (“SEC”) on March 13, 2015 (the "2014 Form 10-K"). The Condensed Consolidated Financial Statements of Prothena Corporation plc are presented in U.S. dollars, which is the functional currency of the Company. The unaudited condensed consolidated financial statements include the accounts of the Company and its consolidated subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
	Unaudited Interim Financial Information
	The accompanying interim Condensed Consolidated Financial Statements and related disclosures are unaudited, have been prepared on the same basis as the annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for a fair presentation of the results of operations for the periods presented. The year-end condensed balance sheet data was derived from audited financial statements, however certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted. The condensed consolidated results of operations for any interim period are not necessarily indicative of the results to be expected for the full year or for any other future year or interim period.
	Use of Estimates
	The preparation of the Condensed Consolidated Financial Statements in conformity with GAAP requires management to make judgments, estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and related disclosures. On an ongoing basis, management evaluates its estimates, including critical accounting policies or estimates related to revenue recognition, share-based compensation and research and development expenses. The Company bases its estimates on historical experience and on various other market specific and other relevant assumptions that management believes to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Because of the uncertainties inherent in such estimates, actual results may differ materially from these estimates.
	Significant Accounting Policies
	There were no significant changes to the accounting policies during the three months ended March 31, 2015, from the significant accounting policies described in Note 2 of the "Notes to Consolidated Financial Statement" in the 2014 Form 10-K.
	Recent Accounting Pronouncements
	In May 2014, the FASB issued Accounting Standards Update 2014-09 (ASU 2014-09), Revenue from Contracts with Customers. ASU 2014-09 supersedes the revenue recognition requirements in Revenue Recognition (Topic 605), and requires entities to recognize revenue in a way that depicts the transfer of promised goods and services to customers in an amount that reflects the consideration to which the entity expects to be entitled to in exchange for those goods or services. ASU 2014-09 is effective for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period, which for the Company is January 1, 2017. Early adoption is not permitted. The standard permits the use of either retrospective or cumulative effect transition method. The Company is currently evaluating the potential impact the adoption of ASU 2014-09 will have on its consolidated financial statements. The Company has not yet selected a transition method nor has it determined the effect of the standard on its ongoing financial reporting.

Fair_Value_Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements
	The Company measures certain financial assets and liabilities at fair value on a recurring basis, including cash equivalents. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or a liability. A three-tier fair value hierarchy is established as a basis for considering such assumptions and for inputs used in the valuation methodologies in measuring fair value:
	Level 1 —    Observable inputs such as quoted prices (unadjusted) for identical assets or liabilities in active markets.
	Level 2 —	Include other inputs that are based upon quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active, and model-based valuation techniques for which all significant inputs are observable in the market or can be derived from observable market data. Where applicable, these models project future cash flows and discount the future amounts to a present value using market-based observable inputs including interest rate curves, foreign exchange rates, and credit ratings.
	Level 3 —	Unobservable inputs that are supported by little or no market activities, which would require the Company to develop its own assumptions.
	The fair value hierarchy also requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. The carrying amounts of certain financial instruments, such as cash equivalents, accounts receivable, accounts payable and accrued liabilities, approximate fair value due to their relatively short maturities, and low market interest rates, if applicable.
	Based on the fair value hierarchy, the Company classifies its cash equivalents within Level 1. This is because the Company values its cash equivalents using quoted market prices. The Company’s Level 1 securities consist of $248.2 million and $262.5 million in money market funds included in cash and cash equivalents at March 31, 2015 and December 31, 2014, respectively.

Composition_of_Certain_Balance

Composition of Certain Balance Sheet Items	3 Months Ended
	Mar. 31, 2015
Composition of Certain Balance Sheet Items [Abstract]
Composition of Certain Balance Sheet Items	Composition of Certain Balance Sheet Items
	Property and Equipment, net
	Property and equipment, net consisted of the following (in thousands):
		March 31,			December 31,
		2015			2014
	Machinery and equipment	$	5,505		$	5,481
	Leasehold improvements	2,217			2,214
	Purchased computer software	138			137
		7,860			7,832
	Less: accumulated depreciation and amortization	(4,904		)	(4,711		)
	Property and equipment, net	$	2,956		$	3,121
	Depreciation expense was $0.2 million and $0.2 million for the three months ended March 31, 2015 and 2014, respectively.
	Other Current Liabilities
	Other current liabilities consisted of the following (in thousands):
		March 31,			December 31,
		2015			2014
	Payroll and related expenses	$	2,784		$	3,138
	Professional services	390			1,169
	Deferred rent	181			172
	Other	537			496
	Other current liabilities	$	3,892		$	4,975

Net_Loss_Per_Ordinary_Share

Net Loss Per Ordinary Share	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Net Loss Per Ordinary Share	Net income (loss) Per Ordinary Share
	Basic net loss per ordinary 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 ordinary share would include the dilutive effect of ordinary shares potentially issuable upon the exercise of stock options outstanding. However, potentially issuable ordinary shares are not used in computing diluted net loss per ordinary share as their effect would be anti-dilutive due to the loss recorded during the three months ended March 31, 2015, and therefore diluted net loss per share is equal to basic net loss per share. During the three months ended March 31, 2014, diluted net income per ordinary share is computed by giving effect to all dilutive potential ordinary shares including options.
	Net income (loss) per ordinary share was determined as follows (in thousands, except per share amounts):
		Three Months Ended March 31,
		2015				2014
	Numerator:
	Net income (loss)	$	(15,202	)		$	17,852
	Denominator (basic):
	Weighted-average ordinary shares outstanding	27,401				21,884
	Denominator (diluted):
	Weighted-average ordinary shares outstanding	27,401				21,884
	Dilutive stock options outstanding	—				1,058
	Net weighted average ordinary shares outstanding	27,401				22,942
	Net income (loss) per share attributable to holders of ordinary shares:
	Basic net income (loss) per share	$	(0.55	)		$	0.82
	Diluted net income (loss) per share	$	(0.55	)		$	0.78
	The equivalent ordinary shares not included in diluted net income (loss) per share because their effect would be anti-dilutive are as follows (in thousands):
		Three Months Ended March 31,
		2015			2014
	Stock options to purchase ordinary shares	3,312			390

Commitment_and_Contingencies

Commitment and Contingencies	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitment and Contingencies	Commitments and Contingencies
	Operating Lease
	In December 2014, the Company entered into a noncancelable operating sublease agreement with a third party to sublease a portion of this leased facility. This sublease agreement has a three-year term which commenced in January 2015 (with options to extend for another year). The Company recognized a loss of $0.4 million in the three months ended March 31, 2015 for the cash difference between the total payments associated with the sublease, including executory costs, and the amount of sublease rental receipts over the sublease term.
	Commitments
	In the normal course of business, the Company enters into various firm purchase commitments primarily related to research and development activities. As of March 31, 2015, the Company had non-cancelable purchase commitments to suppliers for $1.2 million of which $1.1 million is included in accrued current liabilities, and contractual obligations under license agreements of $1.4 million of which $0.1 million is included in accrued current liabilities. These contractual obligations under license agreements exclude future obligations pursuant to the cost-sharing arrangement under the Company's License Agreement with Roche, as the amounts of such obligations, if any, cannot be determined at this time.

Roche_License_Agreement

Roche License Agreement	3 Months Ended
	Mar. 31, 2015
Collaborative Agreement [Abstract]
Roche License Agreements	Roche License Agreement
	In December 2013, the Company entered into the License Agreement with Roche to develop and commercialize certain antibodies that target alpha-synuclein, including PRX002. The License Agreement was evaluated under ASC 605-25, "Multiple Element Arrangements". Under this agreement, the Company recognizes research reimbursement as collaboration revenue as earned. The Company recognized $0.4 million and $0.3 million as collaboration revenue during the three months ended March 31, 2015 and 2014, respectively, for research reimbursement from Roche. Cost sharing payments to Roche are recorded as R&D expenses. The Company recognized $0.5 million and $0.2 million in R&D expense for payments made to Roche during the three months ended March 31, 2015 and 2014, respectively. Reimbursement for development costs from Roche under the cost-sharing arrangement were allocated between license revenue and an offset to R&D expenses based on the relative selling price method until the full allocated consideration of $35.6 million was recognized as license revenue, after which the full reimbursement would be recorded as an offset to R&D expenses. In the three months ended March 31, 2015, the Company reached the full allocated consideration of $35.6 million recognized as license revenue; accordingly, future development revenue will be recorded as an offset to R&D expenses. Reimbursement for development costs from Roche during the three months ended March 31, 2015 was $1.5 million, of which $0.2 million was recognized as collaboration license revenue and $1.3 million was recognized as an offset to R&D expenses. Reimbursement for development costs from Roche during the three months ended March 31, 2014 was $2.0 million, of which $1.8 million was recognized as collaboration license revenue and $0.2 million was recognized as an offset to R&D expenses.
	The Company recognized the $30.0 million upfront payment from Roche as collaboration license revenue in the first quarter of 2014. The Company did not achieve any of the clinical and regulatory milestones under the License Agreement during the three months ended March 31, 2015 and 2014.

Shareholders_Equity

Shareholders' Equity	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Shareholders' Equity	Shareholders' Equity
	Ordinary Shares
	As of March 31, 2015, the Company had 100,000,000 ordinary shares authorized for issuance with a par value of $0.01 per ordinary share and 27,466,407 ordinary shares issued and outstanding. Each ordinary share is entitled to one vote and, on a pro rata basis, to dividends when declared and the remaining assets of the Company in the event of a winding up.
	Euro Deferred Shares
	As of March 31, 2015, the Company had 10,000 Euro Deferred Shares authorized for issuance with a nominal value of €22 per share. No Euro Deferred Shares are outstanding at March 31, 2015. The rights and restrictions attaching to the Euro Deferred Shares rank pari passu with the ordinary shares and are treated as a single class in all respects.
	February 2014 Offering
	In February 2014, Elan Science One Limited ("ESOL"), an indirect wholly owned subsidiary of Perrigo, sold 3,182,253 ordinary shares of Prothena at a price to the public of $26.00 per ordinary share, before the underwriting discount. As a result, ESOL and Perrigo no longer own any ordinary shares of Prothena.

ShareBased_Compensation

Share-Based Compensation	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Compensation	Share-Based Compensation
	Amended and Restated 2012 Long Term Incentive Plan (“LTIP”)
	Employees and consultants of the Company, its subsidiaries and affiliates, as well as members of the Board, are eligible to receive equity awards under the LTIP. The LTIP provides for the grant of stock options, including incentive stock options and nonqualified stock options, stock appreciation rights (“SARS”), restricted shares, restricted share units ("RSUs"), cash or stock-based performance awards and other share-based awards to eligible individuals. Options under the LTIP may be granted for periods up to ten years. All options issued to date have had a ten year life.
	The Company granted 785,550 and 509,000 share options during the three months ended March 31, 2015 and 2014, respectively, under the LTIP. The Company's option awards generally vest over four years. The aggregate number of ordinary shares authorized for issuance under the LTIP is 5,550,000 ordinary shares and as of March 31, 2015, 2,090,470 ordinary shares remain available for grant and options to purchase 3,311,884 ordinary shares granted from the LTIP were outstanding with a weighted-average exercise price of approximately $16.68 per share.
	Share-based Compensation Expense
	The Company estimates the fair value of share-based compensation on the date of grant using an option-pricing model. The Company uses the Black-Scholes model to value share-based compensation, excluding RSUs, which the Company values using the fair market value of its ordinary shares on the date of grant. The Black-Scholes option-pricing model determines the fair value of share-based payment awards based on the share price on the date of grant and is affected by assumptions regarding a number of 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 does not have sufficient historical employee share option exercise data, the simplified method has been used to estimate the expected life of all options. Prior to 2015, the expected volatility was based on historical stock volatilities of several of the Company's publicly traded comparable companies over a period equal to the expected life of the options, as the Company did not have a long enough trading history to use the volatility of its own ordinary shares. Starting in 2015, the expected volatility was based on a combination of historical volatility for the Company's stock and the historical volatilities of several of the Company's publicly traded comparable companies. Although the fair value of share options granted by the Company is estimated by the Black-Scholes model, the estimated fair value may not be indicative of the fair value observed in a willing buyer and seller market transaction.
	As share-based compensation expense recognized in the Condensed Consolidated Financial Statements is based on awards ultimately expected to vest, it has been reduced for estimated forfeitures. Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from estimates. Forfeitures were estimated based on estimated future turnover and historical experience.
	Share-based compensation expense will continue to have an adverse impact on the Company’s results of operations, although it will have no impact on its overall financial position. The amount of unearned share-based compensation currently estimated to be expensed from now through the year 2018 related to unvested share-based payment awards at March 31, 2015 is $28.8 million. The weighted-average period over which the unearned share-based compensation is expected to be recognized is 2.9 years. If there are any modifications or cancellations of the underlying unvested securities, the Company may be required to accelerate, increase or decrease 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 Condensed Consolidated Financial Statements for the three months ended March 31, 2015 and 2014 was based on awards granted under the LTIP. The following table summarizes share-based compensation expense for the periods presented (in thousands):
		Three Months Ended March 31,
		2015			2014
	Research and development(1)	$	758		$	483
	General and administrative	945			860
	Total share-based compensation expense	$	1,703		$	1,343
	_________________
	(1)	Includes $42,000 and $46,000 for the three months ended March 31, 2015 and 2014, respectively, of share-based compensation expense related to options granted to a consultant.
	The fair value of the options granted to employees and non-employee directors during the three months ended March 31, 2015 and 2014 is estimated as of the grant date using the Black-Scholes option-pricing model assuming the weighted-average assumptions listed in the following table:
			Three Months Ended March 31,
			2015	2014
	Expected volatility		76.20%	83.90%
	Risk-free interest rate		1.80%	1.80%
	Expected dividend yield		—%	—%
	Expected life (in years)		6	6
	Weighted average grant date fair value		$18.48	$21.20
	The fair value of employee stock options is being amortized on a straight-line basis over the requisite service period for each award. Each of the inputs discussed above is subjective and generally requires significant management judgment to determine.
	The following table summarizes the Company’s share option activity during the three months ended March 31, 2015:
		Options		Weighted			Weighted	Aggregate
				Average			Average	Intrinsic
				Exercise			Remaining	Value
				Price			Contractual	(in thousands)
							Term (years)
	Outstanding at December 31, 2014	2,612,080		$	13.13		8.45	$	25,298
	Granted	785,550		27.68
	Exercised	(78,402	)	8.24
	Canceled	(7,344	)	22.34
	Outstanding at March 31, 2015	3,311,884		$	16.68		8.62	$	71,088
	Vested and expected to vest at March 31, 2015	3,185,821		$	16.43		8.6	$	69,158
	Vested at March 31, 2015	1,168,041		$	10.19		8.03	$	32,641
	During the three months ended March 31, 2015 and 2014, the total intrinsic value of options exercised was $2.2 million and $1.3 million, respectively, determined as of the date of exercise.

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes
	The major taxing jurisdictions for the Company are Ireland and the U.S. The Company's income tax provision was $266,000 and $151,000 for the three months ended March 31, 2015 and 2014, respectively. The provision for income taxes differs from the statutory tax rate of 12.5% applicable to Ireland primarily due to Irish net operating losses for which a tax provision benefit is not recognized and due to U.S. income taxed at different rates. The income tax provision reflects the estimate of the effective tax rate expected to be applicable for the full year and the Company re-evaluates this estimate each quarter based on its forecasted tax expense for the full year. The effective income tax rate for the three months ended March 31, 2015 does not include a benefit for the U.S. federal research credit as it has not been extended beyond 2014. Jurisdictions with a projected loss for the year where no tax benefit can be recognized are excluded from the estimated annual effective tax rate.
	The Company's deferred tax assets are composed primarily of its Irish subsidiaries' net operating loss carryovers, state net operating loss carryforwards available to reduce future taxable income of the Company's U.S. subsidiary, state tax credit carryforwards, shared-based compensation and other temporary differences. The Company maintains a valuation allowance against certain U.S. federal and state and Irish deferred tax assets. Each reporting period, the Company evaluates the need for a valuation allowance on its deferred tax assets by jurisdiction.
	No provision for income tax in Ireland has been recognized on undistributed earnings of the Company's foreign subsidiaries because the Company considers such earnings to be indefinitely reinvested.

Related_Parties

Related Parties	3 Months Ended
	Mar. 31, 2015
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 ESOL, an indirect wholly owned subsidiary of Perrigo, as of December 31, 2013.
	February 2014 Offering
	In February 2014, ESOL sold 3,182,253 ordinary shares of Prothena at a price to the public of $26.00 per ordinary share, before the underwriting discount. As a result, ESOL and Perrigo no longer owned any ordinary shares of Prothena as of such sale.
	The Company did not receive any of the proceeds from the offering, and the total number of the Company's ordinary shares outstanding did not change as a result of this offering. The Company paid the expenses associated with the sale of these ordinary shares (other than the underwriting discount, fees and disbursements of counsel for the selling shareholder) pursuant to a Subscription and Registration Rights Agreement dated November 8, 2012 by and among the Company, Elan and ESOL.
	Agreements with Elan
	As described elsewhere in these Condensed Consolidated Financial Statements, the results of operations of the Prothena business for the time period prior to the separation include transactions with Elan. The related party revenue recognized by the Company for the three months ended March 31, 2014 consisted of fees arising from R&D services provided to Elan.
	The Company entered into certain agreements with Elan, including the R&D Services Agreement.
	R&D Services Agreement
	In December 2012, as amended in March 2013, the Company entered into a Research and Development Services Agreement (“RDSA”) with Elan pursuant to which the Company provided certain R&D services to Elan. Either party was entitled to terminate the RDSA at any time by notice in writing to the other party if there has been an uncured material breach by the other party or if the other party becomes insolvent or if the other party is in breach of any of its confidentiality obligations under the agreement. This RDSA expired in December 2014 at the end of its two-year term.
	The services provided for under the RDSA included support for the ELND005 program (which include the provision of expert advice and opinion in the areas of nonclinical safety/toxicology and pharmacology, regulatory support for nonclinical sections of pertinent documents, conducting and interpreting externally conducted nonclinical studies, and support in respect of the identification and maintenance of nonclinical expert advisors as required). These services were substantially similar to research services performed by the Company for Elan prior to the separation and distribution.
	The payment terms of the RDSA provided that Elan would pay the Company: (i) a fixed charge of $500,000 per year based on a charge for two of the Company’s employees providing the services at a rate of $250,000 each per annum, (ii) if the $500,000 fixed charge has been paid in any year, a variable charge of $250,000 per year for any additional employee that provides services for such year (calculated pro rata based on the number of days the employee provides services in such year), (iii) research costs including direct overheads, and (iv) a mark-up of 10% applied to the fixed charge, variable charge (if any) and research costs such that the total payment reflects a cost-plus standard. Revenue recognized by the Company three months ended March 31, 2014 included fees arising from R&D services provided to Elan of $138,000. Since the RDSA with Elan terminated in December 2014, the Company did not have any related-party revenue for the three months ended March 31, 2015.

Subsequent_Events

Subsequent Events	3 Months Ended
	Mar. 31, 2015
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events
	April 2015 Public Offering
	In April 2015, the Company completed an underwritten public offering of an aggregate of 3,795,000 of its ordinary shares at a public offering price of $37.00 per ordinary share. The Company received aggregate net proceeds of approximately $131.4 million, after deducting the underwriting discount and estimated offering costs.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Basis of Preparation and Presentation of Financial Information	Basis of Preparation and Presentation of Financial Information
	The accompanying interim Condensed Consolidated Financial Statements have been prepared in accordance with the accounting principles generally accepted in the U.S. (“GAAP”) and with the instructions for Form 10-Q and Regulations S-X statements. Accordingly, they do not include all of the information and notes required for complete financial statements. These interim Condensed Consolidated Financial Statements should be read in conjunction with the Consolidated Financial Statements and Notes thereto contained in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (“SEC”) on March 13, 2015 (the "2014 Form 10-K"). The Condensed Consolidated Financial Statements of Prothena Corporation plc are presented in U.S. dollars, which is the functional currency of the Company. The unaudited condensed consolidated financial statements include the accounts of the Company and its consolidated subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
	Unaudited Interim Financial Information
	The accompanying interim Condensed Consolidated Financial Statements and related disclosures are unaudited, have been prepared on the same basis as the annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for a fair presentation of the results of operations for the periods presented. The year-end condensed balance sheet data was derived from audited financial statements, however certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted. The condensed consolidated results of operations for any interim period are not necessarily indicative of the results to be expected for the full year or for any other future year or interim period.
Use of Estimates	Use of Estimates
	The preparation of the Condensed Consolidated Financial Statements in conformity with GAAP requires management to make judgments, estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and related disclosures. On an ongoing basis, management evaluates its estimates, including critical accounting policies or estimates related to revenue recognition, share-based compensation and research and development expenses. The Company bases its estimates on historical experience and on various other market specific and other relevant assumptions that management believes to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Because of the uncertainties inherent in such estimates, actual results may differ materially from these estimates.
Recent Accounting Pronouncements	Recent Accounting Pronouncements
	In May 2014, the FASB issued Accounting Standards Update 2014-09 (ASU 2014-09), Revenue from Contracts with Customers. ASU 2014-09 supersedes the revenue recognition requirements in Revenue Recognition (Topic 605), and requires entities to recognize revenue in a way that depicts the transfer of promised goods and services to customers in an amount that reflects the consideration to which the entity expects to be entitled to in exchange for those goods or services. ASU 2014-09 is effective for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period, which for the Company is January 1, 2017. Early adoption is not permitted. The standard permits the use of either retrospective or cumulative effect transition method. The Company is currently evaluating the potential impact the adoption of ASU 2014-09 will have on its consolidated financial statements. The Company has not yet selected a transition method nor has it determined the effect of the standard on its ongoing financial reporting.

Composition_of_Certain_Balance1

Composition of Certain Balance Sheet Items (Tables)	3 Months Ended
	Mar. 31, 2015
Composition of Certain Balance Sheet Items [Abstract]
Schedule of Property and Equipment	Property and Equipment, net
	Property and equipment, net consisted of the following (in thousands):
		March 31,			December 31,
		2015			2014
	Machinery and equipment	$	5,505		$	5,481
	Leasehold improvements	2,217			2,214
	Purchased computer software	138			137
		7,860			7,832
	Less: accumulated depreciation and amortization	(4,904		)	(4,711		)
	Property and equipment, net	$	2,956		$	3,121
Schedule of Other Current Liabilities	Other Current Liabilities
	Other current liabilities consisted of the following (in thousands):
		March 31,			December 31,
		2015			2014
	Payroll and related expenses	$	2,784		$	3,138
	Professional services	390			1,169
	Deferred rent	181			172
	Other	537			496
	Other current liabilities	$	3,892		$	4,975

Net_Loss_Per_Ordinary_Share_Ta

Net Loss Per Ordinary Share (Tables)	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Calculation of Basic and Diluted Net Income or Loss Per Ordinary Share	Net income (loss) per ordinary share was determined as follows (in thousands, except per share amounts):
		Three Months Ended March 31,
		2015				2014
	Numerator:
	Net income (loss)	$	(15,202	)		$	17,852
	Denominator (basic):
	Weighted-average ordinary shares outstanding	27,401				21,884
	Denominator (diluted):
	Weighted-average ordinary shares outstanding	27,401				21,884
	Dilutive stock options outstanding	—				1,058
	Net weighted average ordinary shares outstanding	27,401				22,942
	Net income (loss) per share attributable to holders of ordinary shares:
	Basic net income (loss) per share	$	(0.55	)		$	0.82
	Diluted net income (loss) per share	$	(0.55	)		$	0.78
Ordinary Shares Equivalent Not Included in Diluted Net Loss Per Share	The equivalent ordinary shares not included in diluted net income (loss) per share because their effect would be anti-dilutive are as follows (in thousands):
		Three Months Ended March 31,
		2015			2014
	Stock options to purchase ordinary shares	3,312			390

ShareBased_Compensation_Tables

Share-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2015
Summary of Share-Based Compensation Expense	The following table summarizes share-based compensation expense for the periods presented (in thousands):
		Three Months Ended March 31,
		2015			2014
	Research and development(1)	$	758		$	483
	General and administrative	945			860
	Total share-based compensation expense	$	1,703		$	1,343
	_________________
	(1)	Includes $42,000 and $46,000 for the three months ended March 31, 2015 and 2014, respectively, of share-based compensation expense related to options granted to a consultant.
Amended and Restated 2012 Long Term Incentive Plan [Member]
Fair Value of Options Granted	The fair value of the options granted to employees and non-employee directors during the three months ended March 31, 2015 and 2014 is estimated as of the grant date using the Black-Scholes option-pricing model assuming the weighted-average assumptions listed in the following table:
			Three Months Ended March 31,
			2015	2014
	Expected volatility		76.20%	83.90%
	Risk-free interest rate		1.80%	1.80%
	Expected dividend yield		—%	—%
	Expected life (in years)		6	6
	Weighted average grant date fair value		$18.48	$21.20
Summary of Company's Share Option Activity	The following table summarizes the Company’s share option activity during the three months ended March 31, 2015:
		Options		Weighted			Weighted	Aggregate
				Average			Average	Intrinsic
				Exercise			Remaining	Value
				Price			Contractual	(in thousands)
							Term (years)
	Outstanding at December 31, 2014	2,612,080		$	13.13		8.45	$	25,298
	Granted	785,550		27.68
	Exercised	(78,402	)	8.24
	Canceled	(7,344	)	22.34
	Outstanding at March 31, 2015	3,311,884		$	16.68		8.62	$	71,088
	Vested and expected to vest at March 31, 2015	3,185,821		$	16.43		8.6	$	69,158
	Vested at March 31, 2015	1,168,041		$	10.19		8.03	$	32,641

Organization_Additional_Inform

Organization - Additional Information (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit	$63,693	$48,491
Cash and cash equivalents	$279,857	$293,579	$195,052	$176,677

Fair_Value_Measurements_Additi

Fair Value Measurements - Additional Information (Detail) (Level 1 [Member], USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Money market funds at carrying value	$248.20	$262.50

Composition_of_Certain_Balance2

Composition of Certain Balance Sheet Items - Schedule of Property and Equipment (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Composition of Certain Balance Sheet Items [Abstract]
Machinery and equipment	$5,505	$5,481
Leasehold improvements	2,217	2,214
Purchased computer software	138	137
Property and equipment, gross	7,860	7,832
Less: accumulated depreciation and amortization	-4,904	-4,711
Property and equipment, net	$2,956	$3,121

Composition_of_Certain_Balance3

Composition of Certain Balance Sheet Items - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Composition of Certain Balance Sheet Items [Abstract]
Depreciation expense	$0.20	$0.20

Composition_of_Certain_Balance4

Composition of Certain Balance Sheet Items - Schedule of Other Current Liabilities (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Composition of Certain Balance Sheet Items [Abstract]
Payroll and related expenses	$2,784	$3,138
Professional services	390	1,169
Deferred rent	181	172
Other	537	496
Other current liabilities	$3,892	$4,975

Net_Loss_Per_Ordinary_Share_Ca

Net Loss Per Ordinary Share - Calculation of Basic and Diluted Net Income or Loss Per Share (Detail) (USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Numerator:
Net income (loss)	($15,202)	$17,852
Weighted-average ordinary shares outstanding	27,401	21,884
Dilutive stock options outstanding	0	1,058
Net weighted average ordinary shares outstanding	27,401	22,942
Net income (loss) per share attributable to holders of ordinary shares:
Basic net income (loss) per share	($0.55)	$0.82
Diluted net income (loss) per share	($0.55)	$0.78

Net_Loss_Per_Ordinary_Share_Or

Net Loss Per Ordinary Share - Ordinary Shares Equivalent Not Included in Diluted Net Loss Per Share (Detail) (Options to purchase ordinary shares [Member])	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
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 Income (loss) per share	3,312	390

Commitment_and_Contingencies_L

Commitment and Contingencies - Lease Narrative (Details) (USD $)	1 Months Ended	3 Months Ended
In Millions, unless otherwise specified	Jan. 31, 2015	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Sublease term	3 years
Sublease optional extension term	1 year
Loss on sublease		$0.40

Commitment_and_Contingencies_C

Commitment and Contingencies - Commitment Narrative (Details) (USD $)	Mar. 31, 2015
Long-term Purchase Commitment [Line Items]
Purchase obligation	$1,200,000
Accrued current liabilities [Member]
Long-term Purchase Commitment [Line Items]
Commitment to suppliers included in accrued current liabilities	1,100,000
Contractual obligations under license agreements included in accrued current liabilities	100,000
License agreements [Member]
Long-term Purchase Commitment [Line Items]
Contractual obligations under license agreements	$1,432,000

Roche_License_Agreement_Detail

Roche License Agreement (Details) (Roche [Member], USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Collaborative Arrangement [Member]
License Agreement [Line Items]
Cost sharing payments Recognized As Research And Development Expense	$0.50	$0.20
Multiple Element Consideration, Relative Sales Price Method, Amt Allocable to License	35.6
Development Reimbursement	1.5	2
Research Reimbursement [Member]
License Agreement [Line Items]
Collaboration Service Revenue, Research Services	0.4	0.3
Development Costs Reimbursement [Member]
License Agreement [Line Items]
Development Reimbursement Recognized as Collaboration License Revenue	0.2	1.8
Development Reimbursement Recognized As Offset To Research And Development Expense	1.3	0.2
License [Member]
License Agreement [Line Items]
Upfront Payment pursuant to License Agreement		$30

Shareholders_Equity_Ordinary_S

Shareholders' Equity (Ordinary Shares/Euro Deferred Shares) (Details)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2014
	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	27,466,407	27,466,407	27,388,005	27,388,005
Ordinary shares, number of outstanding shares	27,466,407	27,466,407	27,388,005	27,388,005
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 outstanding shares	0	0	0	0

Shareholders_Equity_Shareholde

Shareholders' Equity Shareholders' Equity (Issuance of Ordinary Shares) (Details) (Perrigo, Ordinary Shares, USD $)	1 Months Ended
	Feb. 28, 2014
Perrigo | Ordinary Shares
Shareholders Equity [Line Items]
Shares sold pursuant to underwritten public offering	3,182,253
Share price	$26

ShareBased_Compensation_Additi

Share-Based Compensation - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Stock Option [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Intrinsic value of options exercised	$2.20	1.3
Amended and Restated 2012 Long Term Incentive Plan [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Grant period	10 years
Number of stock options granted to employees	785,550	509,000
Vesting period	4 years
Authorized shares for issuance	5,550,000
Number of shares available for grant	2,090,470
Options outstanding	3,311,884		2,612,080
Weighted average exercise price	$16.68		$13.13
Unearned share-based compensation expected to be expensed through 2017, stock options	$28.80
Expected recognition period of unearned share-based compensation	2 years 11 months 1 day

ShareBased_Compensation_Summar

Share-Based Compensation - Summary of Share-Based Compensation Expense (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$1,703,000		$1,343,000
Consultant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense on option	42,000		46,000
Research and development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	758,000	[1]	483,000	[1]
General and administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$945,000		$860,000
[1]	Includes $42,000 and $46,000 for the three months ended March 31, 2015 and 2014, respectively, of share-based compensation expense related to options granted to a consultant.

ShareBased_Compensation_Fair_V

Share-Based Compensation - Fair Value of Options Granted (Detail) (Amended and Restated 2012 Long Term Incentive Plan [Member], USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Amended and Restated 2012 Long Term Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	76.22%	83.90%
Risk-free interest rate	1.76%	1.80%
Expected dividend yield	0.00%	0.00%
Expected life	6 years	6 years
Weighted average fair value	$18.48	$21.20

Sharebased_Compensation_Shareb

Share-based Compensation - Share-based Compensation Plan - Option Activity (Detail) (Amended and Restated 2012 Long Term Incentive Plan [Member], USD $)	3 Months Ended		12 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Amended and Restated 2012 Long Term Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options, Outstanding beginning balance	2,612,080
Options, granted	785,550	509,000
Options Exercised	-78,402
Options, canceled	-7,344
Options, Outstanding ending balance	3,311,884		2,612,080
Options, Vested and expected to vest ending balance	3,185,821
Options Vested Outstanding Number	1,168,041
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted Average Exercise Price, Outstanding beginning balance	$13.13
Weighted Average Exercise Price, Granted	$27.68
Weighted Average Exercise Price, Exercised	$8.24
Weighted Average Exercise Price, Canceled	$22.34
Weighted Average Exercise Price, Outstanding ending balance	$16.68		$13.13
Weighted Average Exercise Price, Vested and expected to vest ending balance	$16.43
Weighted Average Exercise Price, Options Vested	$10.19
Weighted Average Remaining Contractual Life (Years), Outstanding ending balance	8 years 7 months 13 days		8 years 5 months 12 days
Weighted Average Remaining Contractual Life (Years), Vested and expected to vest ending balance	8 years 7 months 6 days
Weighted Average Remaining Contractual Term, Vested Outstanding	8 years 11 days
Aggregate Intrinsic Value, Outstanding ending balance	$71,088		$25,298
Aggregate Intrinsic Value, Vested and expected to vest ending balance	69,158
Aggregate Intrinsic Value, Options Vested	$32,641

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Income Taxes [Line Items]
Income tax expense (benefit)	$266	$151
Revenue Commissioners, Ireland [Member]
Income Taxes [Line Items]
Federal statutory income tax rate	12.50%	12.50%

Related_Parties_Additional_Inf

Related Parties - Additional Information (Detail) (USD $)	3 Months Ended		1 Months Ended	0 Months Ended	1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2012	Dec. 20, 2012	Feb. 28, 2014
Related Party Transaction [Line Items]
Revenue from Related Parties	$0	$138,000
R&D Services Agreement [Member]
Related Party Transaction [Line Items]
Research and development service agreement, Term (in years)			2 years
Fixed service fee, annual rate			500,000
Fixed service fee per employee, annual rate			250,000
Variable service fee, annual rate			250,000
Percentage of mark-up applied to fixed and variable charge			10.00%
Revenue from Related Parties		$138,000
Elan [Member]
Related Party Transaction [Line Items]
Percentage of ownership interest acquired				18.00%
Issuance of ordinary shares to Elan, shares				3,182,253
Perrigo | Ordinary Shares
Related Party Transaction [Line Items]
Shares sold pursuant to underwritten public offering					3,182,253
Share price					$26

Subsequent_Events_Details

Subsequent Events (Details) (Underwritten Public And Over-Allotment Offering [Member], Subsequent Event [Member], USD $)	1 Months Ended
In Millions, except Share data, unless otherwise specified	Apr. 30, 2015
Subsequent Event [Line Items]
Ordinary shares sold (in dollars per share)	$37
Net proceeds from issuance of ordinary shares in underwritten public offering	$131.40
Ordinary Share [Member]
Subsequent Event [Line Items]
Number of shares issued in transaction	3,795,000