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Securities Sold Under Repurchase Agreements By Collateral Type (Detail) - USD ($)
$ in Millions
Sep. 30, 2015
Dec. 31, 2014
Short Term Debt [Line Items]    
Securities sold under repurchase agreements $ 814 $ 995
Overnight maturity | U.S. Treasury and federal agencies    
Short Term Debt [Line Items]    
Securities sold under repurchase agreements 205 99
Overnight maturity | Agency mortgage-backed securities | Residential mortgage backed securities    
Short Term Debt [Line Items]    
Securities sold under repurchase agreements $ 609 $ 896