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Note Fair Value of financial instruments- Additional Information (Detail) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Letter Of Credit    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Fair Value Disclosure Off balance Sheet Risks Face Amount Asset $ 33 $ 36
Commitments To Extend Credit    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Fair Value Disclosure Off balance Sheet Risks Face Amount Asset $ 7,500 $ 7,600