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, 2020 USD ($) SecurityLoan	Jun. 30, 2019 USD ($) SecurityLoan	Jun. 30, 2020 USD ($) SecurityLoan	Jun. 30, 2019 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,216	2,586	5,014	5,249
Amount Defaulted | $	$ 32	$ 65	$ 115	$ 119
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	330	252	617	486
Amount Defaulted | $	$ 8	$ 4	$ 28	$ 9
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	12	7	28	15
Amount Defaulted | $	$ 6	$ 4	$ 16	$ 10
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	5	15	18	111
Amount Defaulted | $	$ 1	$ 3	$ 2	$ 13
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	82	80	190	227
Amount Defaulted | $	$ 1	$ 1	$ 2	$ 8
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,736	1,922	3,806	3,976
Amount Defaulted | $	$ 9	$ 10	$ 19	$ 19
Total Loans, Excluding Loans Purchased from GNMA Mortgage Pools [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,165	2,276	4,659	4,815
Amount Defaulted | $	$ 25	$ 22	$ 67	$ 59
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	51	310	355	434
Amount Defaulted | $	$ 7	$ 43	$ 48	$ 60