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, 2020 USD ($) SecurityLoan	Mar. 31, 2019 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,798	2,663
Amount Defaulted | $	$ 83	$ 54
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	287	234
Amount Defaulted | $	$ 20	$ 5
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	16	8
Amount Defaulted | $	$ 10	$ 6
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	13	96
Amount Defaulted | $	$ 1	$ 10
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	108	147
Amount Defaulted | $	$ 1	$ 7
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,070	2,054
Amount Defaulted | $	$ 10	$ 9
Total Loans, Excluding Loans Purchased from GNMA Mortgage Pools [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,494	2,539
Amount Defaulted | $	$ 42	$ 37
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	304	124
Amount Defaulted | $	$ 41	$ 17