Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	88	148
Securities available-for-sale gross unrealized losses one year or less	$ (20)	$ (95)
Securities available-for-sale aggregate fair value of investment one year or less	$ 3,381	$ 7,240
Number of Securities available for sale greater than one year \| security	276	154
Securities available-for-sale gross unrealized losses greater than one year	$ (738)	$ (449)
Securities available-for-sale agregate fair value of investment greater than one year	$ 17,393	$ 14,239
Number of securities held-to-maturity in one year or less \| security	215	381
Securities held-to-maturity gross unrealized losses one year or less	$ (22)	$ (53)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 1,453	$ 7,099
Number of securities held-to-maturity greater than one year \| security	1,003	792
Securities held-to-maturity gross unrealized losses greater than one year	$ (277)	$ (96)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 9,111	$ 3,982
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	8	3
Securities available-for-sale gross unrealized losses one year or less	$ (12)	$ (3)
Securities available-for-sale aggregate fair value of investment one year or less	$ 1,013	$ 463
Number of Securities available for sale greater than one year \| security	41	48
Securities available-for-sale gross unrealized losses greater than one year	$ (353)	$ (326)
Securities available-for-sale agregate fair value of investment greater than one year	$ 8,828	$ 10,285
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	55	83
Securities available-for-sale gross unrealized losses one year or less	$ (6)	$ (7)
Securities available-for-sale aggregate fair value of investment one year or less	$ 538	$ 883
Number of Securities available for sale greater than one year \| security	159	77
Securities available-for-sale gross unrealized losses greater than one year	$ (185)	$ (94)
Securities available-for-sale agregate fair value of investment greater than one year	$ 3,698	$ 3,109
Number of securities held-to-maturity in one year or less \| security	103	264
Securities held-to-maturity gross unrealized losses one year or less	$ (8)	$ (5)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 684	$ 1,126
Number of securities held-to-maturity greater than one year \| security	511	284
Securities held-to-maturity gross unrealized losses greater than one year	$ (52)	$ (25)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 1,739	$ 1,020
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	8	52
Securities available-for-sale gross unrealized losses one year or less	$ (2)	$ (85)
Securities available-for-sale aggregate fair value of investment one year or less	$ 106	$ 5,161
Number of Securities available for sale greater than one year \| security	72	23
Securities available-for-sale gross unrealized losses greater than one year	$ (198)	$ (20)
Securities available-for-sale agregate fair value of investment greater than one year	$ 4,825	$ 573
Number of securities held-to-maturity in one year or less \| security	111	116
Securities held-to-maturity gross unrealized losses one year or less	$ (14)	$ (48)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 769	$ 5,973
Number of securities held-to-maturity greater than one year \| security	491	506
Securities held-to-maturity gross unrealized losses greater than one year	$ (225)	$ (71)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 7,372	$ 2,962
Asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	1	0
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	4	4
Securities available-for-sale gross unrealized losses greater than one year	$ (2)	$ (3)
Securities available-for-sale agregate fair value of investment greater than one year	$ 42	$ 51
Foreign Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	16	10
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	$ 1,724	$ 733
Number of Securities available for sale greater than one year \| security	0	1
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	$ 0	$ 44
Equity securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security		0
Securities available-for-sale gross unrealized losses one year or less		$ 0
Securities available-for-sale aggregate fair value of investment one year or less		$ 0
Number of Securities available for sale greater than one year \| security		1
Securities available-for-sale gross unrealized losses greater than one year		$ (6)
Securities available-for-sale agregate fair value of investment greater than one year		$ 177
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	2
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
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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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