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Note 9 - Redeemable Convertible Preferred Stock (Details) - Series D Investors’ Rights (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Series D Investors’ Rights [Abstract]  
Fair value of tranche right liability upon issuance May 13, 2014 $ 6,639prto_FairValueOfInvestorRightsObligations
Issuance costs allocated to tranche right liability (80)prto_IssuanceCostsAllocatedToTrancheRightLiability
Adjustment to fair value recorded in other income during 2014 (5,151)us-gaap_LiabilitiesFairValueAdjustment
Reclassified to equity upon exercise on October 21, 2014 $ (1,408)prto_AdjustmentToAdditionalPaidInCapitalExerciseOfInvestorsRightsAndObligations