Loans and Allowance for Credit Losses - Activity in Allowance for Credit Losses by Portfolio Class (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period		$ 7,695	$ 6,255	$ 7,404	$ 6,155
Change in accounting principle	[1]			(62)
Allowance for acquired credit losses	[2]			127
Provision for credit losses		515	362	1,763	785
Loans charged-off		508	275	1,732	831
Less recoveries of loans charged-off		(88)	(113)	(290)	(346)
Net loan charge-offs (recoveries)		420	162	1,442	485
Balance at end of period		7,790	6,455	7,790	6,455
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period		2,209	1,896	2,163	1,849
Provision for credit losses		(14)	97	169	206
Loans charged-off		110	56	283	164
Less recoveries of loans charged-off		(18)	(29)	(54)	(75)
Net loan charge-offs (recoveries)		92	27	229	89
Balance at end of period		2,103	1,966	2,103	1,966
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period		1,473	973	1,325	1,123
Allowance for acquired credit losses	[2]			127
Provision for credit losses		266	(7)	430	(156)
Loans charged-off		51		205	10
Less recoveries of loans charged-off		(2)	(6)	(13)	(15)
Net loan charge-offs (recoveries)		49	(6)	192	(5)
Balance at end of period		1,690	972	1,690	972
Residential Mortgages [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period		899	658	926	565
Change in accounting principle	[1]			(31)
Provision for credit losses		(49)	38	68	116
Loans charged-off		1	2	126	9
Less recoveries of loans charged-off		(4)	(7)	(16)	(29)
Net loan charge-offs (recoveries)		(3)	(5)	110	(20)
Balance at end of period		853	701	853	701
Other Retail [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period		929	982	970	945
Change in accounting principle	[1]			(4)
Provision for credit losses		27	12	245	94
Loans charged-off		87	56	402	167
Less recoveries of loans charged-off		(25)	(29)	(85)	(95)
Net loan charge-offs (recoveries)		62	27	317	72
Balance at end of period		894	967	894	967
Credit Card [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period		2,185	1,746	2,020	1,673
Change in accounting principle	[1]			(27)
Provision for credit losses		285	222	851	525
Loans charged-off		259	161	716	481
Less recoveries of loans charged-off		(39)	(42)	(122)	(132)
Net loan charge-offs (recoveries)		220	119	594	349
Balance at end of period		$ 2,250	$ 1,849	$ 2,250	$ 1,849

[1] Effective January 1, 2023, the Company adopted accounting guidance which removed the separate recognition and measurement of troubled debt restructurings.

[2] Represents allowance for credit deteriorated and charged-off loans acquired from MUB.