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Commitment and Contingencies - Commitment Narrative (Details) (USD $)
Dec. 31, 2014
Long-term Purchase Commitment [Line Items]  
Purchase Obligation $ 3,197,000us-gaap_PurchaseObligation
Contractual Obligation under License Agreements 1,450,000us-gaap_ContractualObligation [1]
Accrued current liabilities [Member]  
Long-term Purchase Commitment [Line Items]  
Commitment to suppliers included in accrued current liabilities 3,100,000us-gaap_PurchaseCommitmentRemainingMinimumAmountCommitted
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_AccruedLiabilitiesMember
Contractual Obligations under License Agreements included in accrued current liabilities 200,000prta_ContractualObligationsunderLicenseAgreementsincludedinaccruedcurrentliabilities
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_AccruedLiabilitiesMember
License agreements [Member]  
Long-term Purchase Commitment [Line Items]  
Contractual Obligation under License Agreements $ 1,450,000us-gaap_ContractualObligation
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_LicensingAgreementsMember
[1] xcludes future obligations pursuant to the cost-sharing arrangement under the Company's License Agreement with Roche. Amounts of such obligations, if any, cannot be determined at this time.