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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period		$ 6,610	$ 7,890	$ 8,010	$ 4,491
Change in accounting principle	[1]				1,499
Provision for credit losses		(163)	635	(1,160)	3,365
Loans charged-off		266	611	954	1,624
Less recoveries of loans charged-off		(119)	(96)	(404)	(279)
Net loan charge-offs (recoveries)		147	515	550	1,345
Balance at end of period		6,300	8,010	6,300	8,010
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period		1,838	2,645	2,423	1,484
Change in accounting principle	[1]				378
Provision for credit losses		(75)	20	(577)	988
Loans charged-off		40	193	184	406
Less recoveries of loans charged-off		(26)	(15)	(87)	(43)
Net loan charge-offs (recoveries)		14	178	97	363
Balance at end of period		1,749	2,487	1,749	2,487
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period		1,409	1,269	1,544	799
Change in accounting principle	[1]				(122)
Provision for credit losses		(104)	263	(246)	875
Loans charged-off		14	89	28	112
Less recoveries of loans charged-off		(1)	(6)	(22)	(9)
Net loan charge-offs (recoveries)		13	83	6	103
Balance at end of period		1,292	1,449	1,292	1,449
Residential Mortgages [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period		478	633	573	433
Change in accounting principle	[1]				(30)
Provision for credit losses		3	(49)	(107)	179
Loans charged-off		3	4	13	15
Less recoveries of loans charged-off		(13)	(7)	(38)	(20)
Net loan charge-offs (recoveries)		(10)	(3)	(25)	(5)
Balance at end of period		491	587	491	587
Other Retail [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period		994	1,187	1,115	647
Change in accounting principle	[1]				401
Provision for credit losses		36	32	(35)	335
Loans charged-off		55	89	193	316
Less recoveries of loans charged-off		(36)	(33)	(124)	(96)
Net loan charge-offs (recoveries)		19	56	69	220
Balance at end of period		1,011	1,163	1,011	1,163
Credit Card [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period		1,891	2,156	2,355	1,128
Change in accounting principle	[1]				872
Provision for credit losses		(23)	369	(195)	988
Loans charged-off		154	236	536	775
Less recoveries of loans charged-off		(43)	(35)	(133)	(111)
Net loan charge-offs (recoveries)		111	201	403	664
Balance at end of period		$ 1,757	$ 2,324	$ 1,757	$ 2,324

[1] Effective January 1, 2020, the Company adopted accounting guidance which changed impairment recognition of financial instruments to a model that is based on expected losses rather than incurred losses. Upon adoption, the Company increased its allowance for credit losses and reduced retained earnings net of deferred tax liabilities through a cumulative-effect adjustment.