Regulation and Capital Adequacy - Risk-based Capital (Parenthetical) (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Regulation And Capital Adequacy [Abstract]
CECL adoption, percentage of increase in regulatory capital to increase in allowance for credit loss	25.00%
Deferred tax liabilities associated with goodwill	$ 689	$ 667
Deferred tax liabilities associated with identifiable intangible assets	$ 33	$ 37
Subordinated debt maturity period	5 years