Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Detail) $ in Millions	Mar. 31, 2016 USD ($)
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	$ 3,839
Unrealized Losses Less Than 12 Months	(16)
Fair Value 12 Months or Greater	4,317
Unrealized Losses 12 Months or Greater	(45)
Fair Value Total	8,156
Unrealized Losses Total	(61)
Fair Value Less Than 12 Months	8,829
Unrealized Losses Less Than 12 Months	(34)
Fair Value 12 Months or Greater	7,100
Unrealized Losses 12 Months or Greater	(191)
Fair Value Total	15,929
Unrealized Losses Total	(225)
U.S. Treasury and Agencies [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	61
Fair Value Total	61
Fair Value Less Than 12 Months	1,155
Unrealized Losses Less Than 12 Months	(3)
Fair Value 12 Months or Greater	2
Fair Value Total	1,157
Unrealized Losses Total	(3)
Mortgage-Backed Securities Residential [Member] \| Agency [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	3,837
Unrealized Losses Less Than 12 Months	(16)
Fair Value 12 Months or Greater	4,231
Unrealized Losses 12 Months or Greater	(43)
Fair Value Total	8,068
Unrealized Losses Total	(59)
Fair Value Less Than 12 Months	7,387
Unrealized Losses Less Than 12 Months	(28)
Fair Value 12 Months or Greater	6,486
Unrealized Losses 12 Months or Greater	(71)
Fair Value Total	13,873
Unrealized Losses Total	(99)
Mortgage-Backed Securities Residential [Member] \| Non-Agency Prime [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	73
Unrealized Losses Less Than 12 Months	(1)
Fair Value 12 Months or Greater	107
Unrealized Losses 12 Months or Greater	(5)
Fair Value Total	180
Unrealized Losses Total	(6)
Mortgage-Backed Securities Residential [Member] \| Non-Agency Non-Prime [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	38
Unrealized Losses Less Than 12 Months	(1)
Fair Value 12 Months or Greater	13
Unrealized Losses 12 Months or Greater	(3)
Fair Value Total	51
Unrealized Losses Total	(4)
Asset-Backed Securities [Member] \| Asset-Backed Securities Other [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	2
Fair Value 12 Months or Greater	6
Fair Value Total	8
Fair Value 12 Months or Greater	2
Fair Value Total	2
Obligations of State and Political Subdivisions [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	2
Unrealized Losses 12 Months or Greater	(1)
Fair Value Total	2
Unrealized Losses Total	(1)
Fair Value Less Than 12 Months	176
Unrealized Losses Less Than 12 Months	(1)
Fair Value 12 Months or Greater	21
Fair Value Total	197
Unrealized Losses Total	(1)
Other Debt Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	17
Unrealized Losses 12 Months or Greater	(1)
Fair Value Total	17
Unrealized Losses Total	(1)
Corporate Debt Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	399
Unrealized Losses 12 Months or Greater	(97)
Fair Value Total	399
Unrealized Losses Total	(97)
Perpetual Preferred Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	70
Unrealized Losses 12 Months or Greater	(15)
Fair Value Total	70
Unrealized Losses Total	$ (15)

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563

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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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This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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This item represents the fair value of securities categorized as held-to-maturity that have been in a continuous unrealized loss position for less than twelve months.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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