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Accounts Receivable and Contracts in Progress (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Accounts Receivable, Gross $ 64,004,000us-gaap_AccountsReceivableGross $ 40,842,000us-gaap_AccountsReceivableGross  
Unbilled Receivables, Current 7,959,000us-gaap_UnbilledReceivablesCurrent 6,784,000us-gaap_UnbilledReceivablesCurrent  
Accounts Receivable, Net, Current 71,963,000us-gaap_AccountsReceivableNetCurrent 47,626,000us-gaap_AccountsReceivableNetCurrent  
Allowance for Doubtful Accounts Receivable (868,000)us-gaap_AllowanceForDoubtfulAccountsReceivable (1,060,000)us-gaap_AllowanceForDoubtfulAccountsReceivable  
Account and Unbilled Receivables, Net 71,095,000pro_AccountAndUnbilledReceivablesNet 46,566,000pro_AccountAndUnbilledReceivablesNet  
Allowance for Loan and Lease Losses, Provision for Loss, Net (100,000)us-gaap_ProvisionForLoanAndLeaseLosses 400,000us-gaap_ProvisionForLoanAndLeaseLosses (300,000)us-gaap_ProvisionForLoanAndLeaseLosses
Deferred Revenue, Revenue Recognized 349,874,000us-gaap_DeferredRevenueRevenueRecognized 221,905,000us-gaap_DeferredRevenueRevenueRecognized  
Deferred Revenue and Credits (400,349,000)us-gaap_DeferredRevenueAndCredits (260,372,000)us-gaap_DeferredRevenueAndCredits  
Deferred Revenue, Period Increase (Decrease) (50,475,000)us-gaap_DeferredRevenuePeriodIncreaseDecrease (38,467,000)us-gaap_DeferredRevenuePeriodIncreaseDecrease  
Deferred Revenue (58,434,000)us-gaap_DeferredRevenue (45,251,000)us-gaap_DeferredRevenue  
Deferred Maintenance and Support Revenue $ 20,200,000pro_DeferredMaintenanceAndSupportRevenue $ 11,400,000pro_DeferredMaintenanceAndSupportRevenue