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Income Taxes - Additional Information (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Taxes [Line Items]      
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation $ 242,000us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation $ 0us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation $ 0us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation
Income Tax Expense (Benefit) 811,000us-gaap_IncomeTaxExpenseBenefit 415,000us-gaap_IncomeTaxExpenseBenefit 6,000us-gaap_IncomeTaxExpenseBenefit
Valuation allowance (18,815,000)us-gaap_DeferredTaxAssetsValuationAllowance (17,798,000)us-gaap_DeferredTaxAssetsValuationAllowance  
Increase in valuation allowance 1,000,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount    
Tax credits 1,139,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch 1,310,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch  
Irish Tax Rate 12.50%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 12.50%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 12.50%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Ireland [Member]      
Income Taxes [Line Items]      
Irish Tax Rate 12.50%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
   
United States [Member]      
Income Taxes [Line Items]      
Cumulative unremitted earnings 4,100,000us-gaap_CumulativeEarningsDeficit
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_ForeignCountryMember
   
Potential U.S. Withholding Tax Rate 5.00%prta_PotentialU.S.WithholdingTaxRate
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_ForeignCountryMember
   
California [Member]      
Income Taxes [Line Items]      
Operating loss carryforwards 34,100,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
Tax credits 1,100,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
Trading Loss Carryover [Member] | Ireland [Member]      
Income Taxes [Line Items]      
Operating loss carryforwards 100,800,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
/ prta_TypeofLossCarryoversAxis
= prta_TradingLossCarryoverMember
   
Non-Trading Loss Carryover [Member] [Domain] | Ireland [Member]      
Income Taxes [Line Items]      
Operating loss carryforwards $ 5,700,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
/ prta_TypeofLossCarryoversAxis
= prta_NonTradingLossCarryoverMemberDomain