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Available-for-Sale and Held-to-Maturity Securities (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Investment Holdings [Line Items]    
FHLB, restricted stock holdings $ 248us-gaap_FederalHomeLoanBankStock $ 248us-gaap_FederalHomeLoanBankStock
Federal Reserve Bank, restricted stock holdings 352us-gaap_FederalReserveBankStock 352us-gaap_FederalReserveBankStock
Available-for-sale and other securities 26,409us-gaap_AvailableForSaleSecurities [1] 22,408us-gaap_AvailableForSaleSecurities [1]
Interest-Only Mortgage-Backed Securities    
Investment Holdings [Line Items]    
Available-for-sale and other securities $ 158us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InterestOnlyStripMember		 $ 175us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_InterestOnlyStripMember
[1] Amortized cost of $25,475 and $21,677 at March 31, 2015 and December 31, 2014, respectively.