Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Detail) $ in Millions	Sep. 30, 2016 USD ($)
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	$ 3,978
Unrealized Losses Less Than 12 Months	(8)
Fair Value 12 Months or Greater	2,263
Unrealized Losses 12 Months or Greater	(17)
Fair Value Total	6,241
Unrealized Losses Total	(25)
Fair Value Less Than 12 Months	12,118
Unrealized Losses Less Than 12 Months	(46)
Fair Value 12 Months or Greater	3,429
Unrealized Losses 12 Months or Greater	(84)
Fair Value Total	15,547
Unrealized Losses Total	(130)
U.S. Treasury and Agencies [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	250
Unrealized Losses Less Than 12 Months	(1)
Fair Value Total	250
Unrealized Losses Total	(1)
Fair Value Less Than 12 Months	1,611
Unrealized Losses Less Than 12 Months	(3)
Fair Value Total	1,611
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	3,728
Unrealized Losses Less Than 12 Months	(7)
Fair Value 12 Months or Greater	2,237
Unrealized Losses 12 Months or Greater	(16)
Fair Value Total	5,965
Unrealized Losses Total	(23)
Fair Value Less Than 12 Months	9,743
Unrealized Losses Less Than 12 Months	(35)
Fair Value 12 Months or Greater	2,885
Unrealized Losses 12 Months or Greater	(18)
Fair Value Total	12,628
Unrealized Losses Total	(53)
Mortgage-Backed Securities Residential [Member] \| Non-Agency Prime [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	48
Fair Value 12 Months or Greater	91
Unrealized Losses 12 Months or Greater	(4)
Fair Value Total	139
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	19
Fair Value 12 Months or Greater	12
Unrealized Losses 12 Months or Greater	(3)
Fair Value Total	31
Unrealized Losses Total	(3)
Asset-Backed Securities [Member] \| Asset-Backed Securities Other [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	5
Fair Value Total	5
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	3
Unrealized Losses 12 Months or Greater	(1)
Fair Value Total	3
Unrealized Losses Total	(1)
Fair Value Less Than 12 Months	687
Unrealized Losses Less Than 12 Months	(8)
Fair Value 12 Months or Greater	2
Fair Value Total	689
Unrealized Losses Total	(8)
Other Debt Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	18
Fair Value Total	18
Corporate Debt Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	437
Unrealized Losses 12 Months or Greater	(59)
Fair Value Total	437
Unrealized Losses Total	(59)
Commercial [Member] \| Agency [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	10
Fair Value Total	$ 10

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=73527792&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=73527792&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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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