Securities - Securities by Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 192
Amortized Cost of Securities Available for sale, Due after one through five years	18,356
Amortized Cost of Securities Available for sale, Due after five through ten years	1,947
Amortized Cost of Securities Available for sale, Due after ten years	12
Total Amortized Cost of Securities Available for sale	20,507
Fair Value of Securities Available for sale, Due in one year or less	193
Fair Value of Securities Available for sale, Due after one through five years	18,109
Fair Value of Securities Available for sale, Due after five through ten years	1,898
Fair Value of Securities Available for sale, Due after ten years	12
Total Fair Value of Securities Available-for-sale	20,212	$ 14,218
Amortized Cost of Held-to-Maturity Securities, Due in one year or less	54
Amortized Cost of Held-to-Maturity Securities, Due after one through five years	6,892
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	2,678
Amortized Cost of Held-to-Maturity Securities, Due after ten years	608
Held to maturities, Amortized Cost	10,232	4,897
Fair Value of Held-to-Maturity Securities, Due in one year or less	54
Fair Value of Held-to-Maturity Securities, Due after one through five years	6,768
Fair Value of Held-to-Maturity Securities, Due after five through ten years	2,607
Fair Value of Held-to-Maturity Securities, Due after ten years	578
Total Fair Value of Held-to-Maturity Securities	$ 10,007	$ 4,848