Securities - Securities by Maturity (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 276
Amortized Cost of Securities Available for sale, Due after one through five years	13,040
Amortized Cost of Securities Available for sale, Due after five through ten years	46
Amortized Cost of Securities Available for sale, Due after ten years	3
Total Amortized Cost of Securities Available for sale	13,365
Fair Value of Securities Available for sale, Due in one year or less	279
Fair Value of Securities Available for sale, Due after one through five years	13,031
Fair Value of Securities Available for sale, Due after five through ten years	47
Fair Value of Securities Available for sale, Due after ten years	3
Total Fair Value of Securities Available for sale	13,360
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	4,683
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	323
Amortized Cost of Held-to-Maturity Securities, Due after ten years	0
Total Amortized Cost of Held-to-Maturity securities	5,015	4,756
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	4,641
Fair Value of Held-to-Maturity Securities, Due after five through ten years	324
Fair Value of Held-to-Maturity Securities, Due after ten years	0
Total Fair Value of Held-to-Maturity Securities	$ 4,974	$ 4,617