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		6 Months Ended
	Jun. 30, 2021 USD ($) SecurityLoan	Jun. 30, 2020 USD ($) SecurityLoan	Jun. 30, 2021 USD ($) SecurityLoan	Jun. 30, 2020 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,383	2,216	4,764	5,014
Amount Defaulted | $	$ 30	$ 32	$ 71	$ 115
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	327	330	612	617
Amount Defaulted | $	$ 8	$ 8	$ 24	$ 28
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	5	12	12	28
Amount Defaulted | $	$ 1	$ 6	$ 6	$ 16
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	12	5	27	18
Amount Defaulted | $	$ 1	$ 1	$ 3	$ 2
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	191	82	471	190
Amount Defaulted | $	$ 3	$ 1	$ 8	$ 2
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,805	1,736	3,569	3,806
Amount Defaulted | $	$ 11	$ 9	$ 20	$ 19
Total Loans, Excluding Loans Purchased from GNMA Mortgage Pools [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,340	2,165	4,691	4,659
Amount Defaulted | $	$ 24	$ 25	$ 61	$ 67
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	43	51	73	355
Amount Defaulted | $	$ 6	$ 7	$ 10	$ 48