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, 2019 USD ($) SecurityLoan	Sep. 30, 2018 USD ($) SecurityLoan	Sep. 30, 2019 USD ($) SecurityLoan	Sep. 30, 2018 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans	2,644	2,648	7,893	8,060
Amount Defaulted | $	$ 61	$ 117	$ 180	$ 263
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	263	207	749	623
Amount Defaulted | $	$ 9	$ 51	$ 18	$ 63
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	8	10	23	26
Amount Defaulted | $	$ 7	$ 2	$ 17	$ 8
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	13	34	124	148
Amount Defaulted | $	$ 1	$ 4	$ 14	$ 15
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	72	93	299	240
Amount Defaulted | $	$ 1	$ 1	$ 9	$ 3
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	2,025	1,924	6,001	5,893
Amount Defaulted | $	$ 10	$ 9	$ 29	$ 26
Covered Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans				1
Total Loans, Excluding Loans Purchased from GNMA Mortgage Pools [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	2,381	2,268	7,196	6,931
Amount Defaulted | $	$ 28	$ 67	$ 87	$ 115
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	263	380	697	1,129
Amount Defaulted | $	$ 33	$ 50	$ 93	$ 148