Loans and Allowance for Credit Losses - Summary of Loans Modified as TDRs in the Past Twelve Months that have Subsequently Defaulted (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) SecurityLoan	Sep. 30, 2019 USD ($) SecurityLoan	Sep. 30, 2020 USD ($) SecurityLoan	Sep. 30, 2019 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,805	2,644	6,819	7,893
Amount Defaulted | $	$ 49	$ 61	$ 164	$ 180
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	305	263	922	749
Amount Defaulted | $	$ 21	$ 9	$ 49	$ 18
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	5	8	33	23
Amount Defaulted | $	$ 8	$ 7	$ 24	$ 17
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	5	13	23	124
Amount Defaulted | $	$ 2	$ 1	$ 4	$ 14
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	55	72	245	299
Amount Defaulted | $	$ 1	$ 1	$ 3	$ 9
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,363	2,025	5,169	6,001
Amount Defaulted | $	$ 8	$ 10	$ 27	$ 29
Total Loans, Excluding Loans Purchased from GNMA Mortgage Pools [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,733	2,381	6,392	7,196
Amount Defaulted | $	$ 40	$ 28	$ 107	$ 87
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	72	263	427	697
Amount Defaulted | $	$ 9	$ 33	$ 57	$ 93