Securities - Securities by Maturity (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 157
Amortized Cost of Securities Available for sale, Due after one through five years	16,402
Amortized Cost of Securities Available for sale, Due after five through ten years	1,673
Amortized Cost of Securities Available for sale, Due after ten years	21
Total Amortized Cost of Securities Available for sale	18,253
Fair Value of Securities Available for sale, Due in one year or less	159
Fair Value of Securities Available for sale, Due after one through five years	16,200
Fair Value of Securities Available for sale, Due after five through ten years	1,644
Fair Value of Securities Available for sale, Due after ten years	21
Total Fair Value of Securities Available for sale	18,024	$ 20,212
Amortized Cost of Held-to-Maturity Securities, Due in one year or less	64
Amortized Cost of Held-to-Maturity Securities, Due after one through five years	7,637
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	2,329
Amortized Cost of Held-to-Maturity Securities, Due after ten years	608
Held to maturities, Amortized Cost	10,638	10,232
Fair Value of Held-to-Maturity Securities, Due in one year or less	64
Fair Value of Held-to-Maturity Securities, Due after one through five years	7,517
Fair Value of Held-to-Maturity Securities, Due after five through ten years	2,291
Fair Value of Held-to-Maturity Securities, Due after ten years	591
Total Fair Value of Held-to-Maturity Securities	$ 10,463	$ 10,007