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Note 4 - Property and Equipment, Net (Details) - Property and Equipment, Net (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Property, Plant and Equipment [Line Items]    
Property and equipment $ 467us-gaap_PropertyPlantAndEquipmentGross $ 416us-gaap_PropertyPlantAndEquipmentGross
Accumulated Depreciation (384)us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment (354)us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment
Property and equipment, net 83us-gaap_PropertyPlantAndEquipmentNet 62us-gaap_PropertyPlantAndEquipmentNet
Computer Equipment And Software [Member]    
Property, Plant and Equipment [Line Items]    
Property and equipment 135us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= prto_ComputerEquipmentAndSoftwareMember		 96us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= prto_ComputerEquipmentAndSoftwareMember
Furniture and Fixtures [Member]    
Property, Plant and Equipment [Line Items]    
Property and equipment 84us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_FurnitureAndFixturesMember		 84us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_FurnitureAndFixturesMember
Laboratory Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property and equipment $ 248us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= prto_LaboratoryEquipmentMember		 $ 236us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= prto_LaboratoryEquipmentMember