Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Detail) $ in Millions	Sep. 30, 2015 USD ($)
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	$ 5,056
Unrealized Losses Less Than 12 Months	(24)
Fair Value 12 Months or Greater	4,539
Unrealized Losses 12 Months or Greater	(70)
Fair Value Total	9,595
Unrealized Losses Total	(94)
Fair Value Less Than 12 Months	6,862
Unrealized Losses Less Than 12 Months	(35)
Fair Value 12 Months or Greater	7,197
Unrealized Losses 12 Months or Greater	(183)
Fair Value Total	14,059
Unrealized Losses Total	(218)
U.S. Treasury and Agencies [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	339
Unrealized Losses Less Than 12 Months	(1)
Fair Value 12 Months or Greater	64
Unrealized Losses 12 Months or Greater	(1)
Fair Value Total	403
Unrealized Losses Total	(2)
Fair Value Less Than 12 Months	202
Unrealized Losses Less Than 12 Months	(1)
Fair Value 12 Months or Greater	8
Fair Value Total	210
Unrealized Losses Total	(1)
Mortgage-Backed Securities Residential [Member] | Agency [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	4,715
Unrealized Losses Less Than 12 Months	(22)
Fair Value 12 Months or Greater	4,449
Unrealized Losses 12 Months or Greater	(66)
Fair Value Total	9,164
Unrealized Losses Total	(88)
Fair Value Less Than 12 Months	6,276
Unrealized Losses Less Than 12 Months	(27)
Fair Value 12 Months or Greater	6,589
Unrealized Losses 12 Months or Greater	(106)
Fair Value Total	12,865
Unrealized Losses Total	(133)
Mortgage-Backed Securities Residential [Member] | Non-Agency Prime [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	64
Fair Value 12 Months or Greater	75
Unrealized Losses 12 Months or Greater	(3)
Fair Value Total	139
Unrealized Losses Total	(3)
Mortgage-Backed Securities Residential [Member] | Non-Agency Non-Prime [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	10
Fair Value 12 Months or Greater	18
Unrealized Losses 12 Months or Greater	(1)
Fair Value Total	28
Unrealized Losses Total	(1)
Asset-Backed Securities [Member] | Asset-Backed Securities Other [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	6
Unrealized Losses 12 Months or Greater	(1)
Fair Value Total	6
Unrealized Losses Total	(1)
Fair Value Less Than 12 Months	24
Unrealized Losses Less Than 12 Months	(1)
Fair Value 12 Months or Greater	2
Fair Value Total	26
Unrealized Losses Total	(1)
Obligations of State and Political Subdivisions [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	2
Unrealized Losses Less Than 12 Months	(1)
Fair Value Total	2
Unrealized Losses Total	(1)
Fair Value Less Than 12 Months	285
Unrealized Losses Less Than 12 Months	(6)
Fair Value Total	285
Unrealized Losses Total	(6)
Other Debt Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	20
Unrealized Losses 12 Months or Greater	(2)
Fair Value Total	20
Unrealized Losses Total	(2)
Corporate Debt Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	432
Unrealized Losses 12 Months or Greater	(62)
Fair Value Total	432
Unrealized Losses Total	(62)
Perpetual Preferred Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	73
Unrealized Losses 12 Months or Greater	(11)
Fair Value Total	73
Unrealized Losses Total	(11)
Other Investment [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	1
Fair Value Total	$ 1