Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Mar. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	24	88
Securities available-for-sale gross unrealized losses one year or less	$ (15)	$ (20)
Securities available-for-sale aggregate fair value of investment one year or less	$ 2,859	$ 3,381
Number of Securities available for sale greater than one year \| security	287	276
Securities available-for-sale gross unrealized losses greater than one year	$ (534)	$ (738)
Securities available-for-sale agregate fair value of investment greater than one year	$ 17,082	$ 17,393
Number of securities held-to-maturity in one year or less \| security	84	215
Securities held-to-maturity gross unrealized losses one year or less	$ (1)	$ (22)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 369	$ 1,453
Number of securities held-to-maturity greater than one year \| security	1,091	1,003
Securities held-to-maturity gross unrealized losses greater than one year	$ (172)	$ (277)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 9,784	$ 9,111
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	9	8
Securities available-for-sale gross unrealized losses one year or less	$ (15)	$ (12)
Securities available-for-sale aggregate fair value of investment one year or less	$ 1,773	$ 1,013
Number of Securities available for sale greater than one year \| security	42	41
Securities available-for-sale gross unrealized losses greater than one year	$ (297)	$ (353)
Securities available-for-sale agregate fair value of investment greater than one year	$ 8,436	$ 8,828
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	1	55
Securities available-for-sale gross unrealized losses one year or less	$ 0	$ (6)
Securities available-for-sale aggregate fair value of investment one year or less	$ 75	$ 538
Number of Securities available for sale greater than one year \| security	166	159
Securities available-for-sale gross unrealized losses greater than one year	$ (98)	$ (185)
Securities available-for-sale agregate fair value of investment greater than one year	$ 3,816	$ 3,698
Number of securities held-to-maturity in one year or less \| security	15	103
Securities held-to-maturity gross unrealized losses one year or less	$ 0	$ (8)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 3	$ 684
Number of securities held-to-maturity greater than one year \| security	578	511
Securities held-to-maturity gross unrealized losses greater than one year	$ (31)	$ (52)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 2,162	$ 1,739
US Government agency issued or guaranteed [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	4	8
Securities available-for-sale gross unrealized losses one year or less	$ 0	$ (2)
Securities available-for-sale aggregate fair value of investment one year or less	$ 188	$ 106
Number of Securities available for sale greater than one year \| security	73	72
Securities available-for-sale gross unrealized losses greater than one year	$ (135)	$ (198)
Securities available-for-sale agregate fair value of investment greater than one year	$ 4,753	$ 4,825
Number of securities held-to-maturity in one year or less \| security	68	111
Securities held-to-maturity gross unrealized losses one year or less	$ (1)	$ (14)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 366	$ 769
Number of securities held-to-maturity greater than one year \| security	512	491
Securities held-to-maturity gross unrealized losses greater than one year	$ (141)	$ (225)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 7,622	$ 7,372
Asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	0	1
Securities available-for-sale gross unrealized losses one year or less	$ 0	$ 0
Securities available-for-sale aggregate fair value of investment one year or less	$ 0	$ 0
Number of Securities available for sale greater than one year \| security	5	4
Securities available-for-sale gross unrealized losses greater than one year	$ (4)	$ (2)
Securities available-for-sale agregate fair value of investment greater than one year	$ 38	$ 42
Foreign Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	10	16
Securities available-for-sale gross unrealized losses one year or less	$ 0	$ 0
Securities available-for-sale aggregate fair value of investment one year or less	$ 823	$ 1,724
Number of Securities available for sale greater than one year \| security	1	0
Securities available-for-sale gross unrealized losses greater than one year	$ 0	$ 0
Securities available-for-sale agregate fair value of investment greater than one year	$ 39	$ 0
Obligations of U.S. states and political subdivision [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities held-to-maturity in one year or less \| security	1	1
Securities held-to-maturity gross unrealized losses one year or less	$ 0	$ 0
Securities held-to-maturity aggregate fair value of investment one year or less	$ 0	$ 0
Number of securities held-to-maturity greater than one year \| security	1	1
Securities held-to-maturity gross unrealized losses greater than one year	$ 0	$ 0
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 0	$ 0

Held to maturity securities continuous unrealized loss position less than twelve months number of securities.

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Held to maturity securities continuous unrealized loss position twelve months or longer number of securities.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer.

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