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Note Regulatory capital requirements - Additional Information (Detail) - USD ($)
$ in Millions
Dec. 31, 2016
Dec. 31, 2015
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Capital Required to be Well Capitalized to Risk Weighted Assets   10.00%
Tier One Leverage Capital Required to be Well Capitalized to Average Assets   5.00%
Trust Preferred Securities Subject To Phase Out Provision Of Dodd Frank Act   $ 427
Basel III    
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Capital Required to be Well Capitalized to Risk Weighted Assets   10.00%
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets   8.00%
Tier One Leverage Capital Required to be Well Capitalized to Average Assets   5.00%
Trust Preferred Securities That Will Qualify for Tier 2 Treatment $ 427 $ 320
Capital Conservation Buffer   2.50%
Common Equity Tier 1 Based Capital Minimum Requirement Ratio   6.50%