Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Detail) $ in Millions	Sep. 30, 2017 USD ($)
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	$ 22,814
Unrealized Losses Less Than 12 Months	(299)
Fair Value 12 Months or Greater	4,235
Unrealized Losses 12 Months or Greater	(85)
Fair Value Total	27,049
Unrealized Losses Total	(384)
Fair Value Less Than 12 Months	36,130
Unrealized Losses Less Than 12 Months	(414)
Fair Value 12 Months or Greater	9,930
Unrealized Losses 12 Months or Greater	(182)
Fair Value Total	46,060
Unrealized Losses Total	(596)
U.S. Treasury and Agencies [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	2,886
Unrealized Losses Less Than 12 Months	(62)
Fair Value 12 Months or Greater	241
Unrealized Losses 12 Months or Greater	(10)
Fair Value Total	3,127
Unrealized Losses Total	(72)
Fair Value Less Than 12 Months	15,241
Unrealized Losses Less Than 12 Months	(98)
Fair Value 12 Months or Greater	1,389
Unrealized Losses 12 Months or Greater	(22)
Fair Value Total	16,630
Unrealized Losses Total	(120)
Mortgage-Backed Securities Residential [Member] \| Agency [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	19,913
Unrealized Losses Less Than 12 Months	(237)
Fair Value 12 Months or Greater	3,989
Unrealized Losses 12 Months or Greater	(75)
Fair Value Total	23,902
Unrealized Losses Total	(312)
Fair Value Less Than 12 Months	19,025
Unrealized Losses Less Than 12 Months	(281)
Fair Value 12 Months or Greater	7,914
Unrealized Losses 12 Months or Greater	(107)
Fair Value Total	26,939
Unrealized Losses Total	(388)
Commercial [Member] \| Agency [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	6
Fair Value Total	6
Obligations of State and Political Subdivisions [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	1,857
Unrealized Losses Less Than 12 Months	(35)
Fair Value 12 Months or Greater	627
Unrealized Losses 12 Months or Greater	(53)
Fair Value Total	2,484
Unrealized Losses Total	(88)
Other Investment [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	1
Fair Value 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	5
Fair Value Total	5
Other Debt Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	15
Fair Value 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=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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-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563

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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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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