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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
Share data in Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Customer
Dec. 31, 2013
Dec. 31, 2012
Summary Of Significant Accounting Policy [Line Items]      
Major customer revenue 1 billion-plus    
Original maturities 90 days    
Number of customers accounted for revenues or accounts receivable greater than 2% of total 0forr_NumberOfCustomerOwningMoreThanSpecifiedPercentageOfRevenueOrAccountsReceivable    
Annual goodwill impairment test, period November 30    
Goodwill impairment charges $ 0us-gaap_GoodwillImpairmentLoss    
Number of business segment goodwill allocated on a relative fair value basis 3us-gaap_NumberOfOperatingSegments    
Non-current deferred rent liabilities 6,501,000us-gaap_DeferredRentCreditNoncurrent 6,678,000us-gaap_DeferredRentCreditNoncurrent  
Foreign exchange losses 100,000us-gaap_ForeignCurrencyTransactionGainLossRealized 400,000us-gaap_ForeignCurrencyTransactionGainLossRealized 400,000us-gaap_ForeignCurrencyTransactionGainLossRealized
Unamortized fair value stock based compensation 15,800,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized    
Weighted average remaining recognition period 2 years 6 months    
Pre -Tax Expenses [Member]      
Summary Of Significant Accounting Policy [Line Items]      
Additional income tax expense benefit related to out-of-period correction 300,000us-gaap_QuantifyingMisstatementInCurrentYearFinancialStatementsAmount
/ us-gaap_QuantifyingMisstatementInCurrentYearFinancialStatementsByNatureOfErrorAxis
= forr_PreTaxImmaterialExpenseMember		    
Post-Tax Expenses [Member]      
Summary Of Significant Accounting Policy [Line Items]      
Additional income tax expense benefit related to out-of-period correction 200,000us-gaap_QuantifyingMisstatementInCurrentYearFinancialStatementsAmount
/ us-gaap_QuantifyingMisstatementInCurrentYearFinancialStatementsByNatureOfErrorAxis
= forr_PostTaxImmaterialExpenseMember		    
Depreciation Expenses [Member]      
Summary Of Significant Accounting Policy [Line Items]      
Additional income tax expense benefit related to out-of-period correction 400,000us-gaap_QuantifyingMisstatementInCurrentYearFinancialStatementsAmount
/ us-gaap_QuantifyingMisstatementInCurrentYearFinancialStatementsByNatureOfErrorAxis
= forr_DepreciationExpensesMember		    
Other Immaterial Expenses [Member]      
Summary Of Significant Accounting Policy [Line Items]      
Additional income tax expense benefit related to out-of-period correction (100,000)us-gaap_QuantifyingMisstatementInCurrentYearFinancialStatementsAmount
/ us-gaap_QuantifyingMisstatementInCurrentYearFinancialStatementsByNatureOfErrorAxis
= forr_OtherImmaterialExpensesMember		    
Additional Income Tax Expense [Member]      
Summary Of Significant Accounting Policy [Line Items]      
Additional income tax expense benefit related to out-of-period correction $ 500,000us-gaap_QuantifyingMisstatementInCurrentYearFinancialStatementsAmount
/ us-gaap_QuantifyingMisstatementInCurrentYearFinancialStatementsByNatureOfErrorAxis
= forr_IncomeTaxImmaterialExpenseMember		    
Customer Concentration Risk [Member] | Research Service Revenue [Member]      
Summary Of Significant Accounting Policy [Line Items]      
Customer accounted for percentage 3.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember		    
Credit Concentration Risk [Member] | Accounts Receivable [Member]      
Summary Of Significant Accounting Policy [Line Items]      
Customer accounted for percentage 3.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_AccountsReceivableMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember		    
Stock Options [Member]      
Summary Of Significant Accounting Policy [Line Items]      
Anti-dilutive Options to Purchase Shares 0.6us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_EmployeeStockOptionMember		 0.7us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_EmployeeStockOptionMember		 0.8us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_EmployeeStockOptionMember