Organization (Policies)	6 Months Ended
Organization (Policies)	Jun. 30, 2013
Accounting Policies [Abstract]
Description of Business	*Description of Business* Prothena Corporation plc (“Prothena,” the “Company,” “we,” “our” or “us”), a public limited company incorporated in Ireland, is a clinical stage biotechnology company focused on the discovery and development of novel antibodies for the potential treatment of a broad range of diseases that involve protein misfolding or cell adhesion. The Company is focused on the discovery and development of potential therapeutic monoclonal antibodies directed specifically to disease causing proteins. These potential therapies have a broad range of indications including AL and AA forms of amyloidosis (NEOD001), Parkinson’s disease and related synucleinopathies (PRX002) and autoimmune diseases and metastatic cancers (PRX003). The Company has initiated a Phase 1 clinical trial for NEOD001 with the first patient dosing in April 2013. The Phase 1 clinical trial of NEOD001 will evaluate safety and tolerability in AL Amyloidosis patients. 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. Prothena’s business consists of a substantial portion of Elan Corporation plc’s (“Elan”) former drug discovery business platform, including Neotope Biosciences Limited (and its wholly-owned subsidiary Prothena Biosciences Inc) and Onclave Therapeutics Limited, each former wholly-owned subsidiaries of Elan (which for the period prior to separation and distribution are referred to herein as the “Prothena Business”). Effective December 20, 2012, the Prothena Business separated from Elan.
Liquidity and Business Risks	*Liquidity and Business Risks* As of June 30, 2013, the Company had an accumulated deficit of $20.6 million and cash and cash equivalents of $112.5 million. Based on the Company’s current business plans, management believes that the Company’s cash and cash equivalents at June 30, 2013 will be sufficient to meet the Company’s 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 will 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. 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 health care industry.
Use of Estimates	*Use of Estimates* The preparation of the condensed consolidated financial statements in conformity with Generally Accepted Accounting Principles in the United States (“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	*Significant Accounting Policies* There have been no significant changes to the accounting policies during the three months ended June 30, 2013, as compared to the significant accounting policies described in Note 2 of the “Notes to Consolidated Financial Statements” in the Company’s Annual Report for the year ended December 31, 2012 on Form 10-K, which was filed with the Securities and Exchange Commission (“SEC”) on March 29, 2013 (“2012 Form 10-K”) and Note 2 of the “Notes to Condensed Consolidated Financial Statements” in its Quarterly Report on Form 10-Q for the first quarter ended March 31, 2013, which was filed with the SEC on May 15, 2013 (“2013 First Quarter Form 10-Q”).
Basis of Preparation and Presentation of Financial Information	*Basis of Preparation and Presentation of Financial Information* The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with GAAP and applicable rules and regulations of the SEC regarding interim financial reporting. Certain information and note disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. Therefore, these condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes included in the Company’s 2012 Form 10-K. The accompanying condensed consolidated financial statements prior to December 21, 2012 include allocations of direct costs and indirect costs attributable to the Prothena Business operations. The indirect costs included in the Company’s condensed consolidated financial statements relate to certain centralized support functions that were provided by Elan. The centralized support functions provided to the Prothena Business 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 three and six months ended June 30, 2012 were $2.1 million and $4.1 million, respectively. These costs have been allocated to the Prothena Business for the purposes of preparing the consolidated financial statements based on its estimated usage of the resources. 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 have been made on a reasonable basis, but may not necessarily be indicative of the costs that would have been incurred if it had operated on a standalone basis. The condensed consolidated financial statements include the accounts of Prothena and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated. The Condensed Consolidated Balance Sheet as of December 31, 2012, included herein, was derived from the audited financial statements as of that date but does not include all disclosures, including notes required by GAAP. Certain amounts in the condensed consolidated financial statements have been reclassified to conform to the current year presentation. In the opinion of management, the accompanying unaudited condensed consolidated financial statements reflect all normal recurring adjustments necessary to present fairly the financial positions, results of operations and cash flows for the interim periods, but are not necessarily indicative of the results of operations to be anticipated for the year ending December 31, 2013 or any future periods.
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 Condensed Consolidated Statements of Comprehensive Loss is not presented in a separate statement.
Geographical and Customer Concentration	*Geographical and Customer Concentration* The Company’s revenues have been from Ireland for all periods presented, while all of its assets were held in the United States. Revenue recorded in the statements of operations consists of fees earned from the provision of non-clinical 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”), unlike other public companies, the Company is eligible to take advantage of certain exemptions from various reporting requirements that are applicable 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. There have been no new accounting pronouncements or changes to accounting pronouncements during the six months ended June 30, 2013, as compared to the recent accounting pronouncements described in the Company’s 2012 Form 10-K, that are of significance or potential significance to the Company.

Disclosure of accounting policy for reporting when there is a substantial doubt about an entity's ability to continue as a going concern for a reasonable period of time (generally a year from the balance sheet date). Disclose: (a) pertinent conditions and events giving rise to the assessment of substantial doubt about the entity's ability to continue as a going concern for a reasonable period of time, (b) the possible effects of such conditions and events, (c) management's evaluation of the significance of those conditions and events and any mitigating factors, (d) possible discontinuance of operations, (e) management's plans (including relevant prospective financial information), and (f) information about the recoverability or classification of recorded asset amounts or the amounts or classification of liabilities. If management's plans alleviate the substantial doubt about the entity's ability to continue as a going concern, disclosure of the principal conditions and events that initially raised the substantial doubt about the entity's ability to continue as a going concern would be expected to be considered. Disclose whether operations for the current or prior years generated sufficient cash to cover current obligations, whether waivers were obtained from creditors relating to the company's default under the provisions of debt agreements and possible effects of such conditions and events, such as: whether there is a possible need to obtain additional financing (debt or equity) or to liquidate certain holdings to offset future cash flow deficiencies. Disclose appropriate parent company information when parent is dependent upon remittances from subsidiaries to satisfy its obligations.

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