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Note 7 - Stock-based Compensation (Details) (USD $)
3 Months Ended 0 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Jan. 01, 2015
Dec. 31, 2014
Note 7 - Stock-based Compensation (Details) [Line Items]        
Allocated Share-based Compensation Expense (in Dollars) $ 407,000us-gaap_AllocatedShareBasedCompensationExpense $ 17,000us-gaap_AllocatedShareBasedCompensationExpense    
Maximum [Member] | The 2014 Employee Stock Purchase Plan [Member]        
Note 7 - 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     281,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
/ us-gaap_PlanNameAxis
= prto_The2014EmployeeStockPurchasePlanMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
 
The 2014 Employee Stock Purchase Plan [Member]        
Note 7 - Stock-based Compensation (Details) [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 304,991us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= prto_The2014EmployeeStockPurchasePlanMember
    140,500us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= prto_The2014EmployeeStockPurchasePlanMember
Allocated Share-based Compensation Expense (in Dollars) $ 11,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= prto_The2014EmployeeStockPurchasePlanMember