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, 2018 USD ($) SecurityLoan	Sep. 30, 2017 USD ($) SecurityLoan	Sep. 30, 2018 USD ($) SecurityLoan	Sep. 30, 2017 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans	2,648	2,814	8,060	7,974
Amount Defaulted | $	$ 117	$ 91	$ 263	$ 206
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	207	200	623	555
Amount Defaulted | $	$ 51	$ 25	$ 63	$ 49
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	10	10	26	28
Amount Defaulted | $	$ 2	$ 3	$ 8	$ 6
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	34	84	148	251
Amount Defaulted | $	$ 4	$ 7	$ 15	$ 26
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	93	89	240	320
Amount Defaulted | $	$ 1	$ 1	$ 3	$ 4
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	1,924	2,076	5,893	6,107
Amount Defaulted | $	$ 9	$ 9	$ 26	$ 26
Total Loans, Excluding GNMA and Covered Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	2,268	2,459	6,930	7,261
Amount Defaulted | $	$ 67	$ 45	$ 115	$ 111
Covered Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans		1	1	2
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	380	354	1,129	711
Amount Defaulted | $	$ 50	$ 46	$ 148	$ 95