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Note 2 - Summary of Significant Accounting Policies (Details) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended 0 Months Ended
Mar. 31, 2015
Oct. 21, 2014
Oct. 27, 2014
Note 2 - Summary of Significant Accounting Policies (Details) [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period 236us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised    
IPO [Member] | Underwriters [Member]      
Note 2 - Summary of Significant Accounting Policies (Details) [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period   916,500us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_CounterpartyNameAxis
= prto_UnderwritersMember
/ us-gaap_SubsidiarySaleOfStockAxis
= us-gaap_IPOMember
 
IPO [Member]      
Note 2 - Summary of Significant Accounting Policies (Details) [Line Items]      
Stock Issued During Period, Shares, New Issues     7,026,500us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_SubsidiarySaleOfStockAxis
= us-gaap_IPOMember
Share Price (in Dollars per share)     $ 10.00us-gaap_SharePrice
/ us-gaap_SubsidiarySaleOfStockAxis
= us-gaap_IPOMember
Proceeds from Issuance Initial Public Offering (in Dollars)     $ 62.5us-gaap_ProceedsFromIssuanceInitialPublicOffering
/ us-gaap_SubsidiarySaleOfStockAxis
= us-gaap_IPOMember
Payments of Stock Issuance Costs (in Dollars)     $ 7.8us-gaap_PaymentsOfStockIssuanceCosts
/ us-gaap_SubsidiarySaleOfStockAxis
= us-gaap_IPOMember
Conversion of Stock, Shares Converted     8,651,805us-gaap_ConversionOfStockSharesConverted1
/ us-gaap_SubsidiarySaleOfStockAxis
= us-gaap_IPOMember