Securities - Securities by Maturity (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 288
Amortized Cost of Securities Available for sale, Due after one through five years	12,599
Amortized Cost of Securities Available for sale, Due after five through ten years	147
Amortized Cost of Securities Available for sale, Due after ten years	3
Total Amortized Cost of Securities Available for sale	13,037
Fair Value of Securities Available for sale, Due in one year or less	292
Fair Value of Securities Available for sale, Due after one through five years	12,676
Fair Value of Securities Available for sale, Due after five through ten years	149
Fair Value of Securities Available for sale, Due after ten years	3
Total Fair Value of Securities Available for sale	13,120
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	4,504
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	492
Amortized Cost of Held-to-Maturity Securities, Due after ten years	0
Held to maturities, Amortized Cost	5,005	5,015	4,826
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	4,498
Fair Value of Held-to-Maturity Securities, Due after five through ten years	496
Fair Value of Held-to-Maturity Securities, Due after ten years	0
Total Fair Value of Held-to-Maturity Securities	$ 5,003	$ 4,974	$ 4,733