Securities - Securities by Maturity (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 297
Amortized Cost of Securities Available for sale, Due after one through five years	12,659
Amortized Cost of Securities Available for sale, Due after five through ten years	1,329
Amortized Cost of Securities Available for sale, Due after ten years	3
Total Amortized Cost of Securities Available for sale	14,288
Fair Value of Securities Available for sale, Due in one year or less	302
Fair Value of Securities Available for sale, Due after one through five years	12,735
Fair Value of Securities Available for sale, Due after five through ten years	1,336
Fair Value of Securities Available for sale, Due after ten years	3
Total Fair Value of Securities Available for sale	14,376
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	4,310
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	617
Amortized Cost of Held-to-Maturity Securities, Due after ten years	0
Held to maturities, Amortized Cost	4,936	$ 5,015	$ 4,997
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	4,311
Fair Value of Held-to-Maturity Securities, Due after five through ten years	620
Fair Value of Held-to-Maturity Securities, Due after ten years	0
Total Fair Value of Held-to-Maturity Securities	$ 4,940	$ 4,974	$ 4,911