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
	Mar. 31, 2021 USD ($) SecurityLoan	Mar. 31, 2020 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,381	2,798
Amount Defaulted | $	$ 41	$ 83
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	285	287
Amount Defaulted | $	$ 16	$ 20
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	7	16
Amount Defaulted | $	$ 5	$ 10
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	15	13
Amount Defaulted | $	$ 2	$ 1
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	280	108
Amount Defaulted | $	$ 5	$ 1
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,764	2,070
Amount Defaulted | $	$ 9	$ 10
Total Loans, Excluding Loans Purchased from GNMA Mortgage Pools [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,351	2,494
Amount Defaulted | $	$ 37	$ 42
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	30	304
Amount Defaulted | $	$ 4	$ 41