Securities - Details of Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 14,310	$ 13,365
Gross Unrealized Gains	62	132
Gross Unrealized Losses	154	137
Fair Value	14,218	13,360
Held to maturities, Amortized Cost	4,897	5,015
Held to maturities, Gross Unrealized Gains	5	16
Held to maturities, Gross Unrealized Losses	54	57
Held to maturities, Fair Value	4,848	4,974
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21	29
Gross Unrealized Gains		3
Gross Unrealized Losses	1
Fair Value	20	32
Held to maturities, Amortized Cost	20	20
Held to maturities, Fair Value	20	20
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14	22
Gross Unrealized Gains		1
Fair Value	14	23
Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,082	11,310
Gross Unrealized Gains	51	96
Gross Unrealized Losses	138	136
Fair Value	11,995	11,270
Held to maturities, Amortized Cost	4,174	4,755
Held to maturities, Gross Unrealized Gains	5	15
Held to maturities, Gross Unrealized Losses	50	57
Held to maturities, Fair Value	4,129	4,713
Other Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,193	2,004
Gross Unrealized Gains	11	32
Gross Unrealized Losses	15	1
Fair Value	2,189	2,035
Held to maturities, Amortized Cost	703	240
Held to maturities, Gross Unrealized Gains		1
Held to maturities, Gross Unrealized Losses	4
Held to maturities, Fair Value	$ 699	$ 241