Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	$ 3,348
Unrealized Losses Less Than 12 Months	(12)
Fair Value 12 Months or Greater	5,311
Unrealized Losses 12 Months or Greater	(80)
Fair Value Total	8,659
Unrealized Losses Total	(92)
Fair Value Less Than 12 Months	4,688
Unrealized Losses Less Than 12 Months	(19)
Fair Value 12 Months or Greater	7,966
Unrealized Losses 12 Months or Greater	(199)
Fair Value Total	12,654
Unrealized Losses Total	(218)
Mortgage-Backed Securities Residential [Member] \| Agency [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	2,907
Unrealized Losses Less Than 12 Months	(9)
Fair Value 12 Months or Greater	5,171
Unrealized Losses 12 Months or Greater	(76)
Fair Value Total	8,078
Unrealized Losses Total	(85)
Fair Value Less Than 12 Months	4,282
Unrealized Losses Less Than 12 Months	(16)
Fair Value 12 Months or Greater	7,253
Unrealized Losses 12 Months or Greater	(118)
Fair Value Total	11,535
Unrealized Losses Total	(134)
Mortgage-Backed Securities Residential [Member] \| Non-Agency Prime [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	82
Unrealized Losses Less Than 12 Months	(1)
Fair Value 12 Months or Greater	66
Unrealized Losses 12 Months or Greater	(3)
Fair Value Total	148
Unrealized Losses Total	(4)
Mortgage-Backed Securities Residential [Member] \| Non-Agency Non-Prime [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	17
Unrealized Losses Less Than 12 Months
Fair Value 12 Months or Greater	20
Unrealized Losses 12 Months or Greater	(1)
Fair Value Total	37
Unrealized Losses Total	(1)
U.S. Treasury and Agencies [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	439
Unrealized Losses Less Than 12 Months	(2)
Fair Value 12 Months or Greater	114
Unrealized Losses 12 Months or Greater	(2)
Fair Value Total	553
Unrealized Losses Total	(4)
Fair Value Less Than 12 Months	215
Unrealized Losses Less Than 12 Months
Fair Value 12 Months or Greater	108
Unrealized Losses 12 Months or Greater
Fair Value Total	323
Unrealized Losses Total
Asset-Backed Securities [Member] \| Asset-Backed Securities Other [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months
Unrealized Losses Less Than 12 Months
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
Unrealized Losses Less Than 12 Months
Fair Value 12 Months or Greater	3
Unrealized Losses 12 Months or Greater
Fair Value Total	3
Unrealized Losses Total
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 12 Months or Greater
Unrealized Losses 12 Months or Greater
Fair Value Total	2
Unrealized Losses Total	(1)
Fair Value Less Than 12 Months	92
Unrealized Losses Less Than 12 Months	(2)
Fair Value 12 Months or Greater
Unrealized Losses 12 Months or Greater
Fair Value Total	92
Unrealized Losses Total	(2)
Other Debt Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months
Unrealized Losses Less Than 12 Months
Fair Value 12 Months or Greater	20
Unrealized Losses 12 Months or Greater	(1)
Fair Value Total	20
Unrealized Losses Total	(1)
Corporate Debt Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months
Unrealized Losses Less Than 12 Months
Fair Value 12 Months or Greater	440
Unrealized Losses 12 Months or Greater	(68)
Fair Value Total	440
Unrealized Losses Total	(68)
Perpetual Preferred Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months
Unrealized Losses Less Than 12 Months
Fair Value 12 Months or Greater	76
Unrealized Losses 12 Months or Greater	(9)
Fair Value Total	76
Unrealized Losses Total	$ (9)

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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-Publisher FASB

-Name Accounting Standards Codification

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity that have been in a continuous loss position for twelve months or longer.

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