Securities - Securities by Maturity (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 249
Amortized Cost of Securities Available for sale, Due after one through five years	13,456
Amortized Cost of Securities Available for sale, Due after five through ten years	640
Amortized Cost of Securities Available for sale, Due after ten years	1
Total Amortized Cost of Securities Available for sale	14,346
Fair Value of Securities Available for sale, Due in one year or less	252
Fair Value of Securities Available for sale, Due after one through five years	13,649
Fair Value of Securities Available for sale, Due after five through ten years	649
Fair Value of Securities Available for sale, Due after ten years	2
Total Fair Value of Securities Available for sale	14,552
Amortized Cost of Held-to-Maturity Securities, Due in one year or less	86
Amortized Cost of Held-to-Maturity Securities, Due after one through five years	4,035
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	711
Amortized Cost of Held-to-Maturity Securities, Due after ten years	0
Held to maturities, Amortized Cost	4,832	$ 4,897	$ 5,022
Fair Value of Held-to-Maturity Securities, Due in one year or less	87
Fair Value of Held-to-Maturity Securities, Due after one through five years	4,073
Fair Value of Held-to-Maturity Securities, Due after five through ten years	729
Fair Value of Held-to-Maturity Securities, Due after ten years	0
Total Fair Value of Held-to-Maturity Securities	$ 4,889	$ 4,848	$ 4,992