Securities - Details of Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 12,603	$ 11,728	$ 11,569
Gross Unrealized Gains	230	366	393
Gross Unrealized Losses	227
Fair Value	12,606	12,094	11,962
Held to maturities, Amortized Cost	4,835	3,931	4,153
Held to maturities, Gross Unrealized Gains	10	61	59
Held to maturities, Gross Unrealized Losses	115
Held to maturities, Fair Value	4,730	3,992	4,212
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	40	47	52
Gross Unrealized Gains	1	2	2
Gross Unrealized Losses
Fair Value	41	49	54
Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,810	11,148	10,949
Gross Unrealized Gains	194	316	334
Gross Unrealized Losses	225
Fair Value	11,779	11,464	11,283
Held to maturities, Amortized Cost	4,815	3,913	4,135
Held to maturities, Gross Unrealized Gains	10	61	59
Held to maturities, Gross Unrealized Losses	115
Held to maturities, Fair Value	4,710	3,974	4,194
Other Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	733	491	543
Gross Unrealized Gains	31	47	55
Gross Unrealized Losses	2
Fair Value	762	538	598
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20	42	25
Gross Unrealized Gains	4	1	2
Gross Unrealized Losses
Fair Value	24	43	27
Held to maturities, Amortized Cost	20	18	18
Held to maturities, Gross Unrealized Gains
Held to maturities, Gross Unrealized Losses
Held to maturities, Fair Value	$ 20	$ 18	$ 18