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Condensed Consolidated Balance Sheets (Parenthetical)
Dec. 31, 2014
USD ($)
Dec. 31, 2014
EUR (€)
Dec. 31, 2013
USD ($)
Dec. 31, 2013
EUR (€)
Statement of Financial Position [Abstract]        
Euro deferred shares, nominal value (in euros per share)   € 22prta_DeferredSharesParValue   € 22prta_DeferredSharesParValue
Euro deferred shares, number of shares authorized 10,000prta_DeferredSharesSharesAuthorized 10,000prta_DeferredSharesSharesAuthorized 10,000prta_DeferredSharesSharesAuthorized 10,000prta_DeferredSharesSharesAuthorized
Euro deferred shares, number of issued shares 0prta_DeferredSharesShareIssued 0prta_DeferredSharesShareIssued 0prta_DeferredSharesShareIssued 0prta_DeferredSharesShareIssued
Euro deferred shares, number of outstanding shares 0prta_DeferredSharesShareOutstanding 0prta_DeferredSharesShareOutstanding 0prta_DeferredSharesShareOutstanding 0prta_DeferredSharesShareOutstanding
Ordinary shares, par value (in dollars per share) $ 0.01us-gaap_CommonStockParOrStatedValuePerShare   $ 0.01us-gaap_CommonStockParOrStatedValuePerShare  
Ordinary shares, number of authorized shares 100,000,000us-gaap_CommonStockSharesAuthorized 100,000,000us-gaap_CommonStockSharesAuthorized 100,000,000us-gaap_CommonStockSharesAuthorized 100,000,000us-gaap_CommonStockSharesAuthorized
Ordinary shares, number of issued shares 27,388,005us-gaap_CommonStockSharesIssued 27,388,005us-gaap_CommonStockSharesIssued 21,856,261us-gaap_CommonStockSharesIssued 21,856,261us-gaap_CommonStockSharesIssued
Ordinary shares, number of outstanding shares 27,388,005us-gaap_CommonStockSharesOutstanding 27,388,005us-gaap_CommonStockSharesOutstanding 21,856,261us-gaap_CommonStockSharesOutstanding 21,856,261us-gaap_CommonStockSharesOutstanding