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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
Mar. 31, 2015
Level 2 [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral $ 227us-gaap_FinancialInstrumentsOwnedMortgagesMortgageBackedAndAssetBackedSecuritiesAtFairValue
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Fair Value Of Other Financial Instrument [Line Items]  
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AAA - A Rating [Member] | Level 2 [Member] | Available-for-sale Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 208us-gaap_FinancialInstrumentsOwnedMortgagesMortgageBackedAndAssetBackedSecuritiesAtFairValue
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AAA - A Rating [Member] | Level 3 [Member] | Trading Securities [Member]  
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 6us-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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Student loans [Member] | AAA - A Rating [Member] | Level 2 [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 88us-gaap_FinancialInstrumentsOwnedMortgagesMortgageBackedAndAssetBackedSecuritiesAtFairValue
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Fair Value Of Other Financial Instrument [Line Items]  
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Estimate of fair value [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 480us-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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