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Income Taxes - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Operating Loss Carryforwards [Line Items]      
Recognition of previously unrecognized tax benefits 0.00%for_EffectiveIncomeTaxRateReconciliationStateRateChangeDueToPreviouslyUnrecognizedTaxBenefit 15.00%for_EffectiveIncomeTaxRateReconciliationStateRateChangeDueToPreviouslyUnrecognizedTaxBenefit 0.00%for_EffectiveIncomeTaxRateReconciliationStateRateChangeDueToPreviouslyUnrecognizedTaxBenefit
Change in effective tax rate due to acquisition of Credo 0.00%for_EffectiveIncomeTaxRateReconciliationStateRateChangeDueToAcquisition 0.00%for_EffectiveIncomeTaxRateReconciliationStateRateChangeDueToAcquisition (2.00%)for_EffectiveIncomeTaxRateReconciliationStateRateChangeDueToAcquisition
Tax benefits not recognized for book purposes   $ 6,326,000for_UnrecognizedTaxBenefitsIncludingInterestOnIncomeTaxExpenseAndIncomeTaxesAccruedReductionResultingFromLapseOfApplicableStatuteOfLimitations  
Recognized interest accrued related to unrecognized tax benefits 0us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense 75,000us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense 152,000us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense
Accrued interest 0us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued    
Decrease in valuation allowance 9,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount    
Credo [Member]      
Operating Loss Carryforwards [Line Items]      
Oil and gas percentage depletion carryforwards 9,200,000for_OilAndNaturalGasPercentageDepletionCarryforwards
/ us-gaap_BusinessAcquisitionAxis
= for_CredoMember
   
Domestic Tax Authority [Member]      
Operating Loss Carryforwards [Line Items]      
Operating loss carry forwards 7,500,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
   
Expiration date of operating loss carry forwards   Dec. 31, 2031  
State and Local Jurisdiction [Member]      
Operating Loss Carryforwards [Line Items]      
Operating loss carry forwards $ 18,000,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
Minimum [Member] | State and Local Jurisdiction [Member]      
Operating Loss Carryforwards [Line Items]      
Expiration date of operating loss carry forwards   Dec. 31, 2015  
Maximum [Member] | State and Local Jurisdiction [Member]      
Operating Loss Carryforwards [Line Items]      
Expiration date of operating loss carry forwards   Dec. 31, 2034