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Marketable Investments - Summary of Gross Unrealized Losses and Market Value of Available-for-Sale Securities with Unrealized Losses (Detail) (Federal Agency and Corporate Obligations [Member], USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Federal Agency and Corporate Obligations [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities, Less Than 12 Months, Market Value $ 38,175us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= forr_FederalAgencyAndCorporateObligationsMember
$ 30,645us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= forr_FederalAgencyAndCorporateObligationsMember
Available for sale securities, Less Than 12 Months, Unrealized Losses 133us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= forr_FederalAgencyAndCorporateObligationsMember
92us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= forr_FederalAgencyAndCorporateObligationsMember
Available for sale securities, 12 Months or Greater, Market Value 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= forr_FederalAgencyAndCorporateObligationsMember
0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= forr_FederalAgencyAndCorporateObligationsMember
Available for sale securities, 12 Months or Greater, Unrealized Losses $ 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= forr_FederalAgencyAndCorporateObligationsMember
$ 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= forr_FederalAgencyAndCorporateObligationsMember