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Asset-Backed Securities Issued - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Debt Instrument [Line Items]      
Description of interest rate Substantially all ABS issued pay variable rates of interest, which are indexed to one-, three-, or six-month LIBOR.    
Asset-backed Securities      
Debt Instrument [Line Items]      
ABS issued $ 1,545,119,000us-gaap_SecuredDebt
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$ 1,942,962,000us-gaap_SecuredDebt
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Amortization of deferred ABS issuance costs 2,000,000us-gaap_AmortizationOfFinancingCosts
/ us-gaap_DebtInstrumentAxis
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3,000,000us-gaap_AmortizationOfFinancingCosts
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3,000,000us-gaap_AmortizationOfFinancingCosts
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Asset-backed Securities | Contractual maturities of over five years      
Debt Instrument [Line Items]      
ABS issued 1,530,000,000us-gaap_SecuredDebt
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/ us-gaap_InvestmentSecondaryCategorizationAxis
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Contractual maturities of ABS (in years) 5 years    
Asset-backed Securities | Contractual maturities of one to five years      
Debt Instrument [Line Items]      
ABS issued 16,000,000us-gaap_SecuredDebt
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/ us-gaap_InvestmentSecondaryCategorizationAxis
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Contractual maturities of ABS, minimum (in years) 1 year    
Contractual maturities of ABS, maximum (in years) 5 years    
Asset-backed Securities | Certificates With Principal Value      
Debt Instrument [Line Items]      
Principal value 1,555,413,000us-gaap_DebtInstrumentFaceAmount
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1,956,951,000us-gaap_DebtInstrumentFaceAmount
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Asset-backed Securities | Certificates With Principal Value | Contractual maturities of over five years      
Debt Instrument [Line Items]      
Principal value 1,540,000,000us-gaap_DebtInstrumentFaceAmount
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/ us-gaap_InvestmentSecondaryCategorizationAxis
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Asset-backed Securities | Certificates With Principal Value | Contractual maturities of one to five years      
Debt Instrument [Line Items]      
Principal value $ 16,000,000us-gaap_DebtInstrumentFaceAmount
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/ us-gaap_InvestmentSecondaryCategorizationAxis
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