Commitments (Standby and Commercial Letters of Credit, Conditional Commitments Issued to Guarantee the Performance of a Customer to a Third Party) (Parenthetical) (Detail) - Letters of Credit $ in Millions	Dec. 31, 2017 USD ($)
Line of Credit Facility
Commitments	$ 2,185
Less Than One Year From The Balance Sheet Date
Line of Credit Facility
Commitments	1,170	[1]
Less Than One Year From The Balance Sheet Date | Commercial | Commercial customers to facilitate trade payments in U.S. dollars and foreign currencies
Line of Credit Facility
Commitments	7
More than One and within Five Years from Balance Sheet Date
Line of Credit Facility
Commitments	999	[1]
More than One and within Five Years from Balance Sheet Date | Commercial | Commercial customers to facilitate trade payments in U.S. dollars and foreign currencies
Line of Credit Facility
Commitments	1
More than Five Years from Balance Sheet Date
Line of Credit Facility
Commitments	$ 16

[1] (a) Includes $ 7 and $ 1 issued on behalf of commercial customers to facilitate trade payments in U.S. dollars and foreign currencies which expire less than 1 year and between 1 - 5 years, respectively.