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Property and Equipment, net (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Gross $ 31,873,000us-gaap_PropertyPlantAndEquipmentGross $ 27,539,000us-gaap_PropertyPlantAndEquipmentGross  
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment (16,085,000)us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment (11,952,000)us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment  
Property, Plant and Equipment, Net 15,788,000us-gaap_PropertyPlantAndEquipmentNet 15,587,000us-gaap_PropertyPlantAndEquipmentNet  
Depreciation 5,000,000us-gaap_Depreciation 4,189,000us-gaap_Depreciation 2,188,000us-gaap_Depreciation
Disposal of Property Plant and Equipment   1,500,000pro_DisposalOfPropertyPlantAndEquipment 800,000pro_DisposalOfPropertyPlantAndEquipment
Full Depreciated Assets in Use 6,700,000pro_FullDepreciatedAssetsInUse 3,800,000pro_FullDepreciatedAssetsInUse  
Internal-use software development costs capitalized 2,305,000pro_InternalUseSoftwareDevelopmentCostsCapitalized 2,874,000pro_InternalUseSoftwareDevelopmentCostsCapitalized 2,013,000pro_InternalUseSoftwareDevelopmentCostsCapitalized
Internal Use Software Developed, Subject To Amortization 1,600,000pro_InternalUseSoftwareDevelopedSubjectToAmortization 1,700,000pro_InternalUseSoftwareDevelopedSubjectToAmortization  
Capitalized Computer Software, Amortization 200,000us-gaap_CapitalizedComputerSoftwareAmortization 500,000us-gaap_CapitalizedComputerSoftwareAmortization  
Tangible Asset Impairment Charges 4,040,000us-gaap_TangibleAssetImpairmentCharges 0us-gaap_TangibleAssetImpairmentCharges 0us-gaap_TangibleAssetImpairmentCharges
Furniture and Fixtures [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Gross 2,874,000us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_FurnitureAndFixturesMember
2,423,000us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_FurnitureAndFixturesMember
 
Furniture and Fixtures [Member] | Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 7 years    
Furniture and Fixtures [Member] | Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 10 years    
Computer Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Gross 15,662,000us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ComputerEquipmentMember
10,857,000us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ComputerEquipmentMember
 
Computer Equipment [Member] | Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 3 years    
Computer Equipment [Member] | Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 5 years    
Software [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Gross 5,068,000us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ComputerSoftwareIntangibleAssetMember
4,322,000us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ComputerSoftwareIntangibleAssetMember
 
Software [Member] | Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 2 years    
Software [Member] | Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 6 years    
Software Development [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 3 years    
Property, Plant and Equipment, Gross 2,639,000us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_SoftwareDevelopmentMember
5,064,000us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_SoftwareDevelopmentMember
 
Leasehold Improvements [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Estimated Useful Lives Shorter of lease term or useful life    
Property, Plant and Equipment, Gross 5,625,000us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LeaseholdImprovementsMember
4,568,000us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LeaseholdImprovementsMember
 
Construction in Progress [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Gross 5,000us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
305,000us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
 
Cloud-based product offerings [Member]      
Property, Plant and Equipment [Line Items]      
Internal-use software development costs capitalized $ 2,500,000pro_InternalUseSoftwareDevelopmentCostsCapitalized
/ pro_InternalUseSoftwareAxis
= pro_CloudBasedProductOfferingsMember
$ 3,100,000pro_InternalUseSoftwareDevelopmentCostsCapitalized
/ pro_InternalUseSoftwareAxis
= pro_CloudBasedProductOfferingsMember