Condensed Consolidated Statements of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Collaboration Revenue	$ 50,320prta_CollaborationRevenue	$ 0prta_CollaborationRevenue	$ 0prta_CollaborationRevenue
Revenues—related party	534us-gaap_RevenueFromRelatedParties	676us-gaap_RevenueFromRelatedParties	2,658us-gaap_RevenueFromRelatedParties
Revenues	50,854us-gaap_Revenues	676us-gaap_Revenues	2,658us-gaap_Revenues
Operating expenses:
Research and development	38,452us-gaap_ResearchAndDevelopmentExpense	26,052us-gaap_ResearchAndDevelopmentExpense	34,139us-gaap_ResearchAndDevelopmentExpense
General and administrative	19,051us-gaap_GeneralAndAdministrativeExpense	15,051us-gaap_GeneralAndAdministrativeExpense	9,929us-gaap_GeneralAndAdministrativeExpense
Total operating expenses	57,503us-gaap_OperatingExpenses	41,103us-gaap_OperatingExpenses	44,068us-gaap_OperatingExpenses
Loss from operations	(6,649)us-gaap_OperatingIncomeLoss	(40,427)us-gaap_OperatingIncomeLoss	(41,410)us-gaap_OperatingIncomeLoss
Other income (expense):
Interest income	79	71	5
Other income (expense), net	231us-gaap_OtherNonoperatingIncomeExpense	(225)us-gaap_OtherNonoperatingIncomeExpense	0us-gaap_OtherNonoperatingIncomeExpense
Total other income (expense)	310us-gaap_NonoperatingIncomeExpense	(154)us-gaap_NonoperatingIncomeExpense	5us-gaap_NonoperatingIncomeExpense
Loss before income taxes	(6,339)	(40,581)	(41,405)
Provision for income taxes	811us-gaap_IncomeTaxExpenseBenefit	415us-gaap_IncomeTaxExpenseBenefit	6us-gaap_IncomeTaxExpenseBenefit
Net loss	$ (7,150)us-gaap_NetIncomeLoss	$ (40,996)us-gaap_NetIncomeLoss	$ (41,411)us-gaap_NetIncomeLoss
Basic and diluted net loss per share (in dollars per share)	$ (0.29)us-gaap_EarningsPerShareBasicAndDiluted	$ (2.20)us-gaap_EarningsPerShareBasicAndDiluted	$ (2.84)us-gaap_EarningsPerShareBasicAndDiluted
Shares used to compute basic and diluted net loss per share (in shares)	24,672	18,615	14,593

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The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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