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Note 11 - Stock-based Compensation (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Note 11 - Stock-based Compensation (Details) [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term 4 years 6 months    
Allocated Share-based Compensation Expense $ 459,000us-gaap_AllocatedShareBasedCompensationExpense $ 155,000us-gaap_AllocatedShareBasedCompensationExpense $ 110,000us-gaap_AllocatedShareBasedCompensationExpense
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in Shares) 659,865us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross 3,150us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value (in Dollars per share) $ 4.07us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue $ 16.50us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value 200,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue 100,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue  
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options 2,100,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions    
Employee Stock Option [Member] | Non-Employees [Member]      
Note 11 - Stock-based Compensation (Details) [Line Items]      
Allocated Share-based Compensation Expense 4,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_TitleOfIndividualAxis
= prto_NonEmployeesMember
32,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_TitleOfIndividualAxis
= prto_NonEmployeesMember
9,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_TitleOfIndividualAxis
= prto_NonEmployeesMember
Employee Stock Option [Member]      
Note 11 - Stock-based Compensation (Details) [Line Items]      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition   3 years 36 days  
Share-based Compensation Arrangement by Share-based Payment Award, Plan Modification, Number of Employees Affected   1us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationNumberOfEmployeesAffected
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Share-based Compensation Arrangement by Share-based Payment Award Accelerated Compensation Cost   100,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Minimum [Member] | The Plans [Member]      
Note 11 - Stock-based Compensation (Details) [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years    
Maximum [Member] | The Plans [Member]      
Note 11 - Stock-based Compensation (Details) [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 4 years    
Maximum [Member] | The 2014 Employee Stock Purchase Plan [Member]      
Note 11 - Stock-based Compensation (Details) [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award Number of Annual Additional Shares Percentage 1.00%prto_SharebasedCompensationArrangementBySharebasedPaymentAwardNumberOfAnnualAdditionalSharesPercentage
/ us-gaap_PlanNameAxis
= prto_The2014EmployeeStockPurchasePlanMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized (in Shares) 281,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
/ us-gaap_PlanNameAxis
= prto_The2014EmployeeStockPurchasePlanMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Non-Employees [Member]      
Note 11 - Stock-based Compensation (Details) [Line Items]      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options $ 1,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions
/ us-gaap_TitleOfIndividualAxis
= prto_NonEmployeesMember
   
The 2014 Plan [Member]      
Note 11 - Stock-based Compensation (Details) [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in Shares) 704,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= prto_Ther2014PlanMember
   
Share-based Compensation Arrangement by Share-based Payment Award Number of Annual Additional Shares Percentage 4.00%prto_SharebasedCompensationArrangementBySharebasedPaymentAwardNumberOfAnnualAdditionalSharesPercentage
/ us-gaap_PlanNameAxis
= prto_Ther2014PlanMember
   
The 2006 Plan [Member]      
Note 11 - Stock-based Compensation (Details) [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term 10 years    
The 2014 Employee Stock Purchase Plan [Member]      
Note 11 - Stock-based Compensation (Details) [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in Shares) 140,500us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= prto_The2014EmployeeStockPurchasePlanMember