Summary of Significant Accounting Policies	6 Months Ended
Jun. 30, 2013
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of 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* 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 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* 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* 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.

2. Summary of 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

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 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

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

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.