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Net Income per Share Additional Information (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Nov. 27, 2013
Six Percent Tangible Equity Units [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Interest Rate Percentage   6.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= for_A6TangibleEquityUnitsMember
Convertible Debt [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Interest Rate Percentage 3.75%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= us-gaap_ConvertibleDebtMember
 
Conversion price of Convertible Notes 24.49us-gaap_DebtInstrumentConvertibleConversionPrice1
/ us-gaap_DebtInstrumentAxis
= us-gaap_ConvertibleDebtMember
 
Minimum [Member] | Six Percent Tangible Equity Units [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Debt And Equity Instruments, shares to be Issued 6,547,800for_DebtAndEquityInstrumentssharestobeIssued
/ us-gaap_DebtInstrumentAxis
= for_A6TangibleEquityUnitsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
 
Maximum [Member] | Six Percent Tangible Equity Units [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Debt And Equity Instruments, shares to be Issued 7,857,000for_DebtAndEquityInstrumentssharestobeIssued
/ us-gaap_DebtInstrumentAxis
= for_A6TangibleEquityUnitsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember