Securities - Available for Sale Securities (Unrealized Loss Position) (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Gross unrealized losses	$ 23	$ 57	$ 96
Fair value less than 12 months, total temporarily impaired securities	1,520	4,028	2,705
Gross unrealized losses less than 12 months, total temporarily impaired securities	5	63	21
Fair value 12 months or longer, total temporarily impaired securities	4,706	5,006	6,804
Gross unrealized losses 12 months or longer, total temporarily impaired securities	81	131	274
Fair value, total temporarily impaired securities	6,226	9,034	9,509
Gross unrealized losses, total temporarily impaired securities	86	194	295
Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	770	3,019	605
Gross unrealized losses, less than 12 months	3	52	7
Fair value, 12 months or longer	3,348	2,932	4,379
Gross unrealized losses, 12 months or longer	59	84	189
Fair Value	4,118	5,951	4,984
Gross unrealized losses, Total	62	136	196
Held-to-maturity Securities, Fair Value less than 12 months	214	1,005	1,288
Held-to-maturity Securities, Gross unrealized losses, less than 12 months	1	11	13
Held-to-maturity Securities, Fair Value 12 months or longer	1,355	1,994	2,342
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer	22	46	83
Held-to-maturity Securities, Fair Value	1,569	2,999	3,630
Held-to-maturity Securities, Gross unrealized losses	23	57	96
Other Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	420		811
Gross unrealized losses, less than 12 months	1		1
Fair value, 12 months or longer		78	81
Gross unrealized losses, 12 months or longer		1	2
Fair Value	420	78	892
Gross unrealized losses, Total	1	1	3
Held-to-maturity Securities, Fair Value less than 12 months	110
Held-to-maturity Securities, Fair Value	110
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	2	4	1
Fair value, 12 months or longer	3	2	2
Fair Value	5	$ 6	$ 3
Held-to-maturity Securities, Fair Value less than 12 months	4
Held-to-maturity Securities, Fair Value	$ 4