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Note 7 - Option to Acquire Company (Details) - Option to Acquire Company
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Business Acquisition [Line Items]    
Expected dividends 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
Expected term (years) 6 years 5 years 346 days
Risk-free rate 1.88%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate 1.03%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
Major Pharmaceutical Entity [Member]    
Business Acquisition [Line Items]    
Expected volatility 76.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_BusinessAcquisitionAxis
= prto_MajorPharmaceuticalEntityMember
 
Expected option expiration date Jun. 30, 2013  
Expected dividends 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_BusinessAcquisitionAxis
= prto_MajorPharmaceuticalEntityMember
 
Expected term (years) 4 years 124 days  
Risk-free rate 1.80%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_BusinessAcquisitionAxis
= prto_MajorPharmaceuticalEntityMember