Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Detail) $ in Millions	Mar. 31, 2018 USD ($)
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	$ 22,218
Unrealized Losses Less Than 12 Months	(490)
Fair Value 12 Months or Greater	16,869
Unrealized Losses 12 Months or Greater	(757)
Fair Value Total	39,087
Unrealized Losses Total	(1,247)
Fair Value Less Than 12 Months	26,794
Unrealized Losses Less Than 12 Months	(535)
Fair Value 12 Months or Greater	28,973
Unrealized Losses 12 Months or Greater	(1,028)
Fair Value Total	55,767
Unrealized Losses Total	(1,563)
U.S. Treasury and Agencies [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	2,261
Unrealized Losses Less Than 12 Months	(42)
Fair Value 12 Months or Greater	2,557
Unrealized Losses 12 Months or Greater	(139)
Fair Value Total	4,818
Unrealized Losses Total	(181)
Fair Value Less Than 12 Months	13,116
Unrealized Losses Less Than 12 Months	(253)
Fair Value 12 Months or Greater	8,747
Unrealized Losses 12 Months or Greater	(205)
Fair Value Total	21,863
Unrealized Losses Total	(458)
Residential Mortgage-Backed Securities [Member] \| Agency [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	19,957
Unrealized Losses Less Than 12 Months	(448)
Fair Value 12 Months or Greater	14,298
Unrealized Losses 12 Months or Greater	(618)
Fair Value Total	34,255
Unrealized Losses Total	(1,066)
Fair Value Less Than 12 Months	11,235
Unrealized Losses Less Than 12 Months	(236)
Fair Value 12 Months or Greater	18,977
Unrealized Losses 12 Months or Greater	(732)
Fair Value Total	30,212
Unrealized Losses Total	(968)
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	2,437
Unrealized Losses Less Than 12 Months	(46)
Fair Value 12 Months or Greater	1,249
Unrealized Losses 12 Months or Greater	(91)
Fair Value Total	3,686
Unrealized Losses Total	(137)
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
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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=82897076&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
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