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Other Short-Term Borrowings (Securities Sold Under Repurchase Agreements By Collateral Type) (Detail) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Short Term Debt    
Securities sold under repurchase agreements $ 656 $ 661
Overnight maturity | U.S. Treasury and federal agencies    
Short Term Debt    
Securities sold under repurchase agreements 0 0
Overnight maturity | Agency mortgage-backed securities | Residential mortgage backed securities    
Short Term Debt    
Securities sold under repurchase agreements $ 656 $ 661