Securities - Securities by Maturity (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 93
Amortized Cost of Securities Available for sale, Due after one through five years	12,365
Amortized Cost of Securities Available for sale, Due after five through ten years	6,655
Amortized Cost of Securities Available for sale, Due after ten years	12
Amortized Cost	19,125	$ 18,546
Fair Value of Securities Available for sale, Due in one year or less	93
Fair Value of Securities Available for sale, Due after one through five years	11,846
Fair Value of Securities Available for sale, Due after five through ten years	6,390
Fair Value of Securities Available for sale, Due after ten years	12
Total Fair Value of Securities Available For Sale	18,341	18,139
Amortized Cost of Held-to-Maturity Securities, Due in one year or less	31
Amortized Cost of Held-to-Maturity Securities, Due after one through five years	6,181
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	5,657
Amortized Cost of Held-to-Maturity Securities, Due after ten years	0
Amortized Cost	11,869	11,830
Fair Value of Held-to-Maturity Securities, Due in one year or less	30
Fair Value of Held-to-Maturity Securities, Due after one through five years	5,910
Fair Value of Held-to-Maturity Securities, Due after five through ten years	5,321
Fair Value of Held-to-Maturity Securities, Due after ten years	0
Total Fair Value of Held-to-Maturity Securities	$ 11,261	$ 11,565