Securities - Securities by Maturity (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 190
Amortized Cost of Securities Available for sale, Due after one through five years	16,032
Amortized Cost of Securities Available for sale, Due after five through ten years	3,010
Amortized Cost of Securities Available for sale, Due after ten years	10
Total Amortized Cost of Securities Available for sale	19,242
Fair Value of Securities Available for sale, Due in one year or less	192
Fair Value of Securities Available for sale, Due after one through five years	15,825
Fair Value of Securities Available for sale, Due after five through ten years	2,985
Fair Value of Securities Available for sale, Due after ten years	10
Total Fair Value of Securities Available for sale	19,012	$ 20,212
Amortized Cost of Held-to-Maturity Securities, Due in one year or less	76
Amortized Cost of Held-to-Maturity Securities, Due after one through five years	7,506
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	2,087
Amortized Cost of Held-to-Maturity Securities, Due after ten years	607
Held to maturities, Amortized Cost	10,276	10,232
Fair Value of Held-to-Maturity Securities, Due in one year or less	76
Fair Value of Held-to-Maturity Securities, Due after one through five years	7,386
Fair Value of Held-to-Maturity Securities, Due after five through ten years	2,054
Fair Value of Held-to-Maturity Securities, Due after ten years	593
Total Fair Value of Held-to-Maturity Securities	$ 10,109	$ 10,007