Securities - Securities by Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 283
Amortized Cost of Securities Available for sale, Due after one through five years	12,834
Amortized Cost of Securities Available for sale, Due after five through ten years	1,190
Amortized Cost of Securities Available for sale, Due after ten years	3
Total Amortized Cost of Securities Available for sale	14,310
Fair Value of Securities Available for sale, Due in one year or less	287
Fair Value of Securities Available for sale, Due after one through five years	12,757
Fair Value of Securities Available for sale, Due after five through ten years	1,171
Fair Value of Securities Available for sale, Due after ten years	3
Total Fair Value of Securities Available for sale	14,218
Amortized Cost of Held-to-Maturity Securities, Due in one year or less	9
Amortized Cost of Held-to-Maturity Securities, Due after one through five years	4,185
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	645
Amortized Cost of Held-to-Maturity Securities, Due after ten years	58
Held to maturities, Amortized Cost	4,897	$ 5,015
Fair Value of Held-to-Maturity Securities, Due in one year or less	9
Fair Value of Held-to-Maturity Securities, Due after one through five years	4,140
Fair Value of Held-to-Maturity Securities, Due after five through ten years	641
Fair Value of Held-to-Maturity Securities, Due after ten years	58
Total Fair Value of Held-to-Maturity Securities	$ 4,848	$ 4,974