Loans - Summary of Changes in Allowance for Credit Losses (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Allowance for loan losses
Balance, beginning of period	$ 1,441	$ 1,066
Impact of CECL adoption	727
Beginning balance, adjusted	2,168
Net charge-offs	(731)	(490)
Provision	2,502	990
Other	(225)	(125)
Balance, end of period	$ 3,714	$ 1,441
Allowance ratio	3.70%	1.60%
Net charge-off ratio	1.00%	0.60%
Allowance for losses on lending commitments
Balance, beginning of period	$ 361	$ 286
Impact of CECL adoption	(48)
Beginning balance, adjusted	313
Provision	302	75
Balance, end of period	615	361
Wholesale [Member]
Allowance for loan losses
Balance, beginning of period	879	658
Impact of CECL adoption	452
Beginning balance, adjusted	1,331
Net charge-offs	(501)	(121)
Provision	1,977	469
Other	(225)	(127)
Balance, end of period	$ 2,582	$ 879
Allowance ratio	2.80%	1.10%
Net charge-off ratio	0.70%	0.20%
Allowance for losses on lending commitments
Balance, beginning of period	$ 361	$ 286
Impact of CECL adoption	(48)
Beginning balance, adjusted	313
Provision	302	75
Balance, end of period	615	361
Consumer [Member]
Allowance for loan losses
Balance, beginning of period	393	292
Impact of CECL adoption	444
Beginning balance, adjusted	837
Net charge-offs	(230)	(317)
Provision	525	418
Balance, end of period	$ 1,132	$ 393
Allowance ratio	16.10%	6.00%
Net charge-off ratio	4.50%	6.20%
PCI [Member]
Allowance for loan losses
Balance, beginning of period	$ 169	$ 116
Impact of CECL adoption	$ (169)
Net charge-offs		(52)
Provision		103
Other		2
Balance, end of period		$ 169
Allowance ratio		10.40%
Net charge-off ratio		3.20%