Securities - Securities by Maturity (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 113
Amortized Cost of Securities Available for sale, Due after one through five years	12,105
Amortized Cost of Securities Available for sale, Due after five through ten years	5,823
Amortized Cost of Securities Available for sale, Due after ten years	16
Amortized Cost	18,057	$ 18,546
Fair Value of Securities Available for sale, Due in one year or less	113
Fair Value of Securities Available for sale, Due after one through five years	11,602
Fair Value of Securities Available for sale, Due after five through ten years	5,636
Fair Value of Securities Available for sale, Due after ten years	16
Total Fair Value of Securities Available For Sale	17,367	18,139
Amortized Cost of Held-to-Maturity Securities, Due in one year or less	43
Amortized Cost of Held-to-Maturity Securities, Due after one through five years	6,653
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	5,581
Amortized Cost of Held-to-Maturity Securities, Due after ten years	0
Amortized Cost	12,277	11,830
Fair Value of Held-to-Maturity Securities, Due in one year or less	42
Fair Value of Held-to-Maturity Securities, Due after one through five years	6,410
Fair Value of Held-to-Maturity Securities, Due after five through ten years	5,283
Fair Value of Held-to-Maturity Securities, Due after ten years	0
Total Fair Value of Held-to-Maturity Securities	$ 11,735	$ 11,565