Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Detail) $ in Millions	Mar. 31, 2017 USD ($)
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	$ 30,228
Unrealized Losses Less Than 12 Months	(523)
Fair Value 12 Months or Greater	2,147
Unrealized Losses 12 Months or Greater	(62)
Fair Value Total	32,375
Unrealized Losses Total	(585)
Fair Value Less Than 12 Months	45,312
Unrealized Losses Less Than 12 Months	(807)
Fair Value 12 Months or Greater	4,154
Unrealized Losses 12 Months or Greater	(96)
Fair Value Total	49,466
Unrealized Losses Total	(903)
U.S. Treasury and Agencies [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	3,841
Unrealized Losses Less Than 12 Months	(113)
Fair Value Total	3,841
Unrealized Losses Total	(113)
Fair Value Less Than 12 Months	15,616
Unrealized Losses Less Than 12 Months	(169)
Fair Value Total	15,616
Unrealized Losses Total	(169)
Mortgage-Backed Securities Residential [Member] \| Agency [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	26,372
Unrealized Losses Less Than 12 Months	(410)
Fair Value 12 Months or Greater	2,142
Unrealized Losses 12 Months or Greater	(62)
Fair Value Total	28,514
Unrealized Losses Total	(472)
Fair Value Less Than 12 Months	27,465
Unrealized Losses Less Than 12 Months	(486)
Fair Value 12 Months or Greater	4,149
Unrealized Losses 12 Months or Greater	(96)
Fair Value Total	31,614
Unrealized Losses Total	(582)
Commercial [Member] \| Agency [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	9
Fair Value Total	9
Obligations of State and Political Subdivisions [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	2,222
Unrealized Losses Less Than 12 Months	(152)
Fair Value 12 Months or Greater	4
Fair Value Total	2,226
Unrealized Losses Total	(152)
Other Investment [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value 12 Months or Greater	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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-URI http://asc.fasb.org/extlink&oid=73527792&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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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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