Loans and Allowance for Credit Losses - Activity in Allowance for Credit Losses by Portfolio Class (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	$ 8,010	$ 4,491
Change in accounting principle		1,499
Provision for credit losses	(827)	993
Loans charged-off	374	491
Less recoveries of loans charged-off	(151)	(98)
Net loans charged-off (recoveries)	223	393
Balance at end of period	6,960	6,590
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	2,423	1,484
Change in accounting principle		378
Provision for credit losses	(435)	452
Loans charged-off	86	88
Less recoveries of loans charged-off	(30)	(14)
Net loans charged-off (recoveries)	56	74
Balance at end of period	1,932	2,240
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	1,544	799
Change in accounting principle		(122)
Provision for credit losses	(19)	162
Loans charged-off	10
Less recoveries of loans charged-off	(17)	(2)
Net loans charged-off (recoveries)	(7)	(2)
Balance at end of period	1,532	841
Residential Mortgages [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	573	433
Change in accounting principle		(30)
Provision for credit losses	(39)	10
Loans charged-off	5	8
Less recoveries of loans charged-off	(10)	(7)
Net loans charged-off (recoveries)	(5)	1
Balance at end of period	539	412
Other Retail [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	1,115	647
Change in accounting principle		401
Provision for credit losses	(75)	123
Loans charged-off	83	121
Less recoveries of loans charged-off	(48)	(35)
Net loans charged-off (recoveries)	35	86
Balance at end of period	1,005	1,085
Credit Card [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	2,355	1,128
Change in accounting principle		872
Provision for credit losses	(259)	246
Loans charged-off	190	274
Less recoveries of loans charged-off	(46)	(40)
Net loans charged-off (recoveries)	144	234
Balance at end of period	$ 1,952	$ 2,012