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Note 14 - Net Income (Loss) Per Share Attributable to Common Stockholders (Details) - Common Stock Equivalents Exluded From Calculation of Net Loss Per Share
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Antidilutive Securities 9,720us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount 5,764us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount 5,528us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Convertible Preferred Stock [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Antidilutive Securities 7,581us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_StatementClassOfStockAxis
= us-gaap_ConvertiblePreferredStockMember		 [1] 4,254us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_StatementClassOfStockAxis
= us-gaap_ConvertiblePreferredStockMember		 4,254us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_StatementClassOfStockAxis
= us-gaap_ConvertiblePreferredStockMember
Common Stock Warrants [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Antidilutive Securities 660us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= prto_CommonStockWarrantsMember		 [1] 660us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= prto_CommonStockWarrantsMember		 660us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= prto_CommonStockWarrantsMember
Equity Option [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Antidilutive Securities 1,236us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_StockOptionMember		 607us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_StockOptionMember		 614us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_StockOptionMember
Convertible Debt Securities [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Antidilutive Securities 243us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_ConvertibleDebtSecuritiesMember		 [1] 243us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_ConvertibleDebtSecuritiesMember		  
[1] These instruments were either exercised or converted to Common Stock during 2014.