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, 2021 USD ($) SecurityLoan	Sep. 30, 2020 USD ($) SecurityLoan	Sep. 30, 2021 USD ($) SecurityLoan	Sep. 30, 2020 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,490	1,805	7,254	6,819
Amount Defaulted | $	$ 27	$ 49	$ 98	$ 164
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	244	305	856	922
Amount Defaulted | $	$ 5	$ 21	$ 29	$ 49
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	4	5	16	33
Amount Defaulted | $	$ 1	$ 8	$ 7	$ 24
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	20	5	47	23
Amount Defaulted | $	$ 2	$ 2	$ 5	$ 4
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	124	55	595	245
Amount Defaulted | $	$ 2	$ 1	$ 10	$ 3
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,069	1,363	5,638	5,169
Amount Defaulted | $	$ 12	$ 8	$ 32	$ 27
Total Loans, Excluding Loans Purchased from GNMA Mortgage Pools [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,461	1,733	7,152	6,392
Amount Defaulted | $	$ 22	$ 40	$ 83	$ 107
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	29	72	102	427
Amount Defaulted | $	$ 5	$ 9	$ 15	$ 57