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Debt - Debt additional information (Detail)
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Senior Notes [Member]    
Line of Credit Facility [Line Items]    
Interest Rate Percentage 8.50%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		  
Debt Instrument, Maturity Date Jun. 01, 2022  
Six Percent Tangible Equity Units [Member]    
Line of Credit Facility [Line Items]    
Average interest rate 6.00%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= for_A6TangibleEquityUnitsMember		 6.00%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= for_A6TangibleEquityUnitsMember
Debt Instrument, Maturity Date Dec. 15, 2016  
Secured Debt [Member]    
Line of Credit Facility [Line Items]    
Average interest rate 3.18%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SecuredDebtMember		 3.17%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SecuredDebtMember
Other indebtedness [Member]    
Line of Credit Facility [Line Items]    
Variable and fixed interest rates ranging, minimum 2.19%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_OtherDebtSecuritiesMember		  
Variable and fixed interest rates ranging, maximum 5.50%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_OtherDebtSecuritiesMember		  
Convertible Debt [Member]    
Line of Credit Facility [Line Items]    
Interest Rate Percentage 3.75%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= us-gaap_ConvertibleDebtMember		  
Debt Instrument, Maturity Date Mar. 01, 2020