Investment Securities - Gross Unrealized Losses and Fair Value of Company's Investment Securities (Detail) $ in Millions	Jun. 30, 2018 USD ($)
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	$ 23,972
Unrealized Losses Less Than 12 Months	(637)
Fair Value 12 Months or Greater	16,345
Unrealized Losses 12 Months or Greater	(843)
Fair Value Total	40,317
Unrealized Losses Total	(1,480)
Fair Value Less Than 12 Months	28,209
Unrealized Losses Less Than 12 Months	(647)
Fair Value 12 Months or Greater	27,433
Unrealized Losses 12 Months or Greater	(1,117)
Fair Value Total	55,642
Unrealized Losses Total	(1,764)
U.S. Treasury and Agencies [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	2,250
Unrealized Losses Less Than 12 Months	(52)
Fair Value 12 Months or Greater	2,541
Unrealized Losses 12 Months or Greater	(151)
Fair Value Total	4,791
Unrealized Losses Total	(203)
Fair Value Less Than 12 Months	12,332
Unrealized Losses Less Than 12 Months	(294)
Fair Value 12 Months or Greater	8,199
Unrealized Losses 12 Months or Greater	(221)
Fair Value Total	20,531
Unrealized Losses Total	(515)
Residential Mortgage-Backed Securities [Member] \| Agency [Member]
Continuous Gross Unrealized Losses And Fair Value [Abstract]
Fair Value Less Than 12 Months	21,722
Unrealized Losses Less Than 12 Months	(585)
Fair Value 12 Months or Greater	13,791
Unrealized Losses 12 Months or Greater	(692)
Fair Value Total	35,513
Unrealized Losses Total	(1,277)
Fair Value Less Than 12 Months	13,044
Unrealized Losses Less Than 12 Months	(305)
Fair Value 12 Months or Greater	17,989
Unrealized Losses 12 Months or Greater	(808)
Fair Value Total	31,033
Unrealized Losses Total	(1,113)
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	2,827
Unrealized Losses Less Than 12 Months	(48)
Fair Value 12 Months or Greater	1,245
Unrealized Losses 12 Months or Greater	(88)
Fair Value Total	4,072
Unrealized Losses Total	(136)
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	11
Fair Value Total	$ 11

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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-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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