Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investments Securities with Unrealized Losses (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Schedule Of Fair Value And Gross Unrealized Losses [Abstract]
Fair Value Less Than 12 Months	$ 3,924
Unrealized Losses Less Than 12 Months	(22)
Fair Value 12 Months or Greater	108
Unrealized Losses 12 Months or Greater	(18)
Fair Value Total	4,032
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Losses, Total	(40)
Fair Value Less Than 12 Months	7,535
Unrealized Losses Less Than 12 Months	(45)
Fair Value 12 Months or Greater	1,174
Unrealized Losses 12 Months or Greater	(111)
Fair Value Total	8,709
Unrealized Losses Total	(156)
U.S. Treasury and Agencies [Member]
Schedule Of Fair Value And Gross Unrealized Losses [Abstract]
Fair Value Less Than 12 Months	444
Unrealized Losses Less Than 12 Months	(2)
Fair Value Total	444
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Losses, Total	(2)
Fair Value Less Than 12 Months	599
Unrealized Losses Less Than 12 Months	(1)
Fair Value Total	599
Unrealized Losses Total	(1)
Residential [Member] | Residential Agency [Member]
Schedule Of Fair Value And Gross Unrealized Losses [Abstract]
Fair Value Less Than 12 Months	3,478
Unrealized Losses Less Than 12 Months	(20)
Fair Value Total	3,478
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Losses, Total	(20)
Fair Value Less Than 12 Months	6,484
Unrealized Losses Less Than 12 Months	(40)
Fair Value 12 Months or Greater	166
Fair Value Total	6,650
Unrealized Losses Total	(40)
Residential [Member] | Residential Non-agency Prime [Member]
Schedule Of Fair Value And Gross Unrealized Losses [Abstract]
Fair Value Less Than 12 Months	91
Fair Value 12 Months or Greater	254
Unrealized Losses 12 Months or Greater	(12)
Fair Value Total	345
Unrealized Losses Total	(12)
Residential [Member] | Residential Non-agency Non-prime [Member]
Schedule Of Fair Value And Gross Unrealized Losses [Abstract]
Fair Value Less Than 12 Months	22
Fair Value 12 Months or Greater	190
Unrealized Losses 12 Months or Greater	(9)
Fair Value Total	212
Unrealized Losses Total	(9)
Commercial Mortgage-backed securities [Member] | Non-agency [Member]
Schedule Of Fair Value And Gross Unrealized Losses [Abstract]
Fair Value Less Than 12 Months	2
Fair Value Total	2
Asset Backed Securities Other [Member]
Schedule Of Fair Value And Gross Unrealized Losses [Abstract]
Fair Value 12 Months or Greater	9
Unrealized Losses 12 Months or Greater	(3)
Fair Value Total	9
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Losses, Total	(3)
Fair Value 12 Months or Greater	2
Unrealized Losses 12 Months or Greater	(1)
Fair Value Total	2
Unrealized Losses Total	(1)
Corporate debt securities [Member]
Schedule Of Fair Value And Gross Unrealized Losses [Abstract]
Fair Value 12 Months or Greater	99
Unrealized Losses 12 Months or Greater	(15)
Fair Value Total	99
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Losses, Total	(15)
Fair Value 12 Months or Greater	429
Unrealized Losses 12 Months or Greater	(77)
Fair Value Total	429
Unrealized Losses Total	(77)
Obligations of State and Political Subdivisions [Member]
Schedule Of Fair Value And Gross Unrealized Losses [Abstract]
Fair Value Less Than 12 Months	339
Unrealized Losses Less Than 12 Months	(4)
Fair Value 12 Months or Greater	12
Fair Value Total	351
Unrealized Losses Total	(4)
Perpetual preferred securities [Member]
Schedule Of Fair Value And Gross Unrealized Losses [Abstract]
Fair Value 12 Months or Greater	121
Unrealized Losses 12 Months or Greater	(12)
Fair Value Total	121
Unrealized Losses Total	$ (12)