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Stock Option Plans - Summary of Option Activity (Detail) (USD $)
3 Months Ended
Mar. 31, 2015
Number of Shares  
Outstanding at beginning of period (in shares) 748,208us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
Granted (in shares) 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
Exercised (in shares) (66,684)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
Expired/Forfeited (in shares) 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod
Outstanding at end of period (in shares) 681,524us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
Exercisable at end of period (in shares) 314,024us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
Weighted average exercise price per share  
Outstanding at beginning of period (in usd per share) $ 44.79us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
Granted (in usd per share) $ 0.00us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
Exercised (in usd per share) $ 43.12us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice
Expired/Forfeited (in usd per share) $ 0.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice
Outstanding at end of period (in usd per share) $ 44.96us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
Exercisable at end of period (in usd per share) $ 45.22us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice
Aggregate intrinsic value  
Outstanding at beginning of period $ 9,274,028us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue
Granted 0bfs_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodIntrinsicValue
Exercised 969,165us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Expired/Forfeited   
Outstanding at end of period 8,342,475us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue
Exercisable at end of period $ 3,763,237us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1