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Summary of Significant Accounting Policies - Options Granted Under Equity Incentive Plans and Shares Subject to Employee Stock Purchase Plan Valuation Assumptions (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Equity Incentive Plans [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Average risk-free interest rate 1.69%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_PlanNameAxis
= forr_EquityIncentivePlansMember
0.85%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_PlanNameAxis
= forr_EquityIncentivePlansMember
0.85%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_PlanNameAxis
= forr_EquityIncentivePlansMember
Expected dividend yield 1.80%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_PlanNameAxis
= forr_EquityIncentivePlansMember
2.10%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_PlanNameAxis
= forr_EquityIncentivePlansMember
1.70%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_PlanNameAxis
= forr_EquityIncentivePlansMember
Expected life 5 years 1 month 6 days 4 years 10 months 24 days 4 years 6 months
Expected volatility 26.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_PlanNameAxis
= forr_EquityIncentivePlansMember
36.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_PlanNameAxis
= forr_EquityIncentivePlansMember
40.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_PlanNameAxis
= forr_EquityIncentivePlansMember
Weighted average fair value $ 7.91us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_PlanNameAxis
= forr_EquityIncentivePlansMember
$ 9.21us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_PlanNameAxis
= forr_EquityIncentivePlansMember
$ 9.64us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_PlanNameAxis
= forr_EquityIncentivePlansMember
Employee Stock Purchase Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Average risk-free interest rate 0.06%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_PlanNameAxis
= forr_EmployeeStockPurchasePlanMember
0.12%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_PlanNameAxis
= forr_EmployeeStockPurchasePlanMember
0.14%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_PlanNameAxis
= forr_EmployeeStockPurchasePlanMember
Expected dividend yield 2.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_PlanNameAxis
= forr_EmployeeStockPurchasePlanMember
1.90%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_PlanNameAxis
= forr_EmployeeStockPurchasePlanMember
1.70%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_PlanNameAxis
= forr_EmployeeStockPurchasePlanMember
Expected life 6 months 6 months 6 months
Expected volatility 24.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_PlanNameAxis
= forr_EmployeeStockPurchasePlanMember
22.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_PlanNameAxis
= forr_EmployeeStockPurchasePlanMember
31.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_PlanNameAxis
= forr_EmployeeStockPurchasePlanMember
Weighted average fair value $ 7.32us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_PlanNameAxis
= forr_EmployeeStockPurchasePlanMember
$ 6.02us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_PlanNameAxis
= forr_EmployeeStockPurchasePlanMember
$ 6.90us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_PlanNameAxis
= forr_EmployeeStockPurchasePlanMember