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Share-Based Compensation - Fair Value of Options Granted (Detail) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2014
Dec. 31, 2013
Elan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility 60.10%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_PlanNameAxis
= prta_ElanMember
   
Risk-free interest rate 0.90%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_PlanNameAxis
= prta_ElanMember
   
Expected dividend yield 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_PlanNameAxis
= prta_ElanMember
   
Expected life 10 years    
Weighted average fair value $ 6.66us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_PlanNameAxis
= prta_ElanMember
   
Grant period 10 years    
Amended and Restated 2012 Long Term Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility   84.44%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_PlanNameAxis
= prta_TwoThousandTwelveLongTermIncentivePlanMember
84.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_PlanNameAxis
= prta_TwoThousandTwelveLongTermIncentivePlanMember
Risk-free interest rate   1.80%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_PlanNameAxis
= prta_TwoThousandTwelveLongTermIncentivePlanMember
1.20%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_PlanNameAxis
= prta_TwoThousandTwelveLongTermIncentivePlanMember
Expected dividend yield   0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_PlanNameAxis
= prta_TwoThousandTwelveLongTermIncentivePlanMember
0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_PlanNameAxis
= prta_TwoThousandTwelveLongTermIncentivePlanMember
Expected life   6 years 6 years
Weighted average fair value   $ 19.97us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_PlanNameAxis
= prta_TwoThousandTwelveLongTermIncentivePlanMember
$ 5.22us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_PlanNameAxis
= prta_TwoThousandTwelveLongTermIncentivePlanMember
Grant period   10 years  
Minimum [Member] | Elan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected life 4 years 10 months 24 days [1]    
Maximum [Member] | Elan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected life 6 years 9 months 18 days [1]    
[1] The expected life of options granted, as derived from the output of the binomial model, ranged from 4.9 to 6.8 years in 2012. The contractual life of the options, which is not more than 10 years from the date of grant, was used as an input into the binomial model.