Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Detail) $ in Millions	Sep. 30, 2018 USD ($)
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	$ 19,965
Unrealized Losses Less Than 12 Months	(565)
Fair Value 12 Months or Greater	21,198
Unrealized Losses 12 Months or Greater	(1,183)
Fair Value Total	41,163
Unrealized Losses Total	(1,748)
Fair Value Less Than 12 Months	23,426
Unrealized Losses Less Than 12 Months	(596)
Fair Value 12 Months or Greater	32,395
Unrealized Losses 12 Months or Greater	(1,539)
Fair Value Total	55,821
Unrealized Losses Total	(2,135)
U.S. Treasury and Agencies [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	1,867
Unrealized Losses Less Than 12 Months	(43)
Fair Value 12 Months or Greater	2,993
Unrealized Losses 12 Months or Greater	(193)
Fair Value Total	4,860
Unrealized Losses Total	(236)
Fair Value Less Than 12 Months	8,685
Unrealized Losses Less Than 12 Months	(225)
Fair Value 12 Months or Greater	10,848
Unrealized Losses 12 Months or Greater	(371)
Fair Value Total	19,533
Unrealized Losses Total	(596)
Residential Mortgage-Backed Securities [Member] \| Agency [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	18,097
Unrealized Losses Less Than 12 Months	(522)
Fair Value 12 Months or Greater	18,191
Unrealized Losses 12 Months or Greater	(990)
Fair Value Total	36,288
Unrealized Losses Total	(1,512)
Fair Value Less Than 12 Months	10,786
Unrealized Losses Less Than 12 Months	(246)
Fair Value 12 Months or Greater	20,350
Unrealized Losses 12 Months or Greater	(1,033)
Fair Value Total	31,136
Unrealized Losses Total	(1,279)
Commercial [Member] \| Agency [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	5
Fair Value Total	5
Obligations of State and Political Subdivisions [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	3,950
Unrealized Losses Less Than 12 Months	(125)
Fair Value 12 Months or Greater	1,197
Unrealized Losses 12 Months or Greater	(135)
Fair Value Total	5,147
Unrealized Losses Total	(260)
Asset-Backed Securities [Member] \| Other Asset-Backed Securities [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	2
Fair Value Total	2
Obligations of Foreign Governments [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	1
Fair Value Total	1
Other [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	12
Fair Value Total	$ 12

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=82897076&loc=d3e27290-111563

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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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