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Fair Value Measurements - Additional Information Relating To Asset-Backed Securities and Collateralized Debt Obligations (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Level 2 [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral $ 228us-gaap_FinancialInstrumentsOwnedMortgagesMortgageBackedAndAssetBackedSecuritiesAtFairValue
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 0us-gaap_FinancialInstrumentsOwnedMortgagesMortgageBackedAndAssetBackedSecuritiesAtFairValue
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Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 5us-gaap_FinancialInstrumentsOwnedMortgagesMortgageBackedAndAssetBackedSecuritiesAtFairValue
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Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 0us-gaap_FinancialInstrumentsOwnedMortgagesMortgageBackedAndAssetBackedSecuritiesAtFairValue
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Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 0us-gaap_FinancialInstrumentsOwnedMortgagesMortgageBackedAndAssetBackedSecuritiesAtFairValue
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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Fair Value Of Other Financial Instrument [Line Items]  
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