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	12 Months Ended
	Mar. 31, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans	2,649	2,647
Amount Defaulted | $	$ 58	$ 60
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	239	173
Amount Defaulted | $	$ 9	$ 8
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	8	8
Amount Defaulted | $	$ 4	$ 2
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	56	72
Amount Defaulted | $	$ 4	$ 9
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	77	129
Amount Defaulted | $	$ 1	$ 2
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	2,036	2,047
Amount Defaulted | $	$ 9	$ 9
Total Loans, Excluding GNMA and Covered Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	2,416	2,429
Amount Defaulted | $	$ 27	$ 30
Covered Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	1
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans	232	218
Amount Defaulted | $	$ 31	$ 30