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, 2019 USD ($) SecurityLoan	Mar. 31, 2018 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans	2,663	2,649
Amount Defaulted | $	$ 54	$ 58
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	234	239
Amount Defaulted | $	$ 5	$ 9
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	8	8
Amount Defaulted | $	$ 6	$ 4
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	96	56
Amount Defaulted | $	$ 10	$ 4
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	147	77
Amount Defaulted | $	$ 7	$ 1
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	2,054	2,036
Amount Defaulted | $	$ 9	$ 9
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	2,539	2,417
Amount Defaulted | $	$ 37	$ 27
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	124	232
Amount Defaulted | $	$ 17	$ 31