Investment Securities - Investment Securities Held-to-Maturity (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, Amortized Cost	$ 46,285	$ 46,050
Held-to-maturity securities, Unrealized Gains	111	51
Held-to-maturity securities, Unrealized Losses Other-than-Temporary	0	0
Held-to-maturity securities, Unrealized Losses Other	(661)	(1,137)
Held-to-maturity securities, Fair Value	45,735	44,964
U.S. Treasury and Agencies [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, Amortized Cost	4,931	5,102
Held-to-maturity securities, Unrealized Gains	3	2
Held-to-maturity securities, Unrealized Losses Other-than-Temporary	0	0
Held-to-maturity securities, Unrealized Losses Other	(92)	(143)
Held-to-maturity securities, Fair Value	4,842	4,961
Residential Mortgage-Backed Securities [Member] \| Agency [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, Amortized Cost	41,327	40,920
Held-to-maturity securities, Unrealized Gains	103	45
Held-to-maturity securities, Unrealized Losses Other-than-Temporary	0	0
Held-to-maturity securities, Unrealized Losses Other	(569)	(994)
Held-to-maturity securities, Fair Value	40,861	39,971
Asset-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, Amortized Cost	5
Held-to-maturity securities, Fair Value	10
Asset-Backed Securities [Member] \| Collateralized Loan Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, Unrealized Gains	3	1
Held-to-maturity securities, Unrealized Losses Other-than-Temporary	0	0
Held-to-maturity securities, Fair Value	3	1
Asset-Backed Securities [Member] \| Other Asset-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, Amortized Cost	5	5
Held-to-maturity securities, Unrealized Gains	2	2
Held-to-maturity securities, Unrealized Losses Other-than-Temporary	0	0
Held-to-maturity securities, Fair Value	7	7
Obligations of State and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, Amortized Cost	5	6
Held-to-maturity securities, Unrealized Gains		1
Held-to-maturity securities, Unrealized Losses Other-than-Temporary	0	0
Held-to-maturity securities, Fair Value	5	7
Obligations of Foreign Governments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, Amortized Cost	9	9
Held-to-maturity securities, Unrealized Losses Other-than-Temporary	0	0
Held-to-maturity securities, Fair Value	9	9
Other [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, Amortized Cost	8	8
Held-to-maturity securities, Unrealized Losses Other-than-Temporary	0	0
Held-to-maturity securities, Fair Value	$ 8	$ 8

Amount of investment in debt security measured at amortized cost (held-to-maturity).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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