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, 2018 USD ($) SecurityLoan	Jun. 30, 2017 USD ($) SecurityLoan	Jun. 30, 2018 USD ($) SecurityLoan	Jun. 30, 2017 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans	2,763	2,513	5,412	5,160
Amount Defaulted | $	$ 88	$ 55	$ 146	$ 115
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	177	182	416	355
Amount Defaulted | $	$ 3	$ 16	$ 12	$ 24
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	8	10	16	18
Amount Defaulted | $	$ 2	$ 1	$ 6	$ 3
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	58	95	114	167
Amount Defaulted | $	$ 7	$ 10	$ 11	$ 19
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	70	102	147	231
Amount Defaulted | $	$ 1	$ 1	$ 2	$ 3
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	1,933	1,984	3,969	4,031
Amount Defaulted | $	$ 8	$ 8	$ 17	$ 17
Total Loans, Excluding GNMA and Covered Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	2,246	2,373	4,662	4,802
Amount Defaulted | $	$ 21	$ 36	$ 48	$ 66
Covered Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans		1	1	1
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	517	139	749	357
Amount Defaulted | $	$ 67	$ 19	$ 98	$ 49