Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Sep. 30, 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	62	88
Securities available-for-sale gross unrealized losses one year or less	$ (48)	$ (20)
Securities available-for-sale aggregate fair value of investment one year or less	$ 6,905	$ 3,381
Number of Securities available for sale greater than one year \| security	172	276
Securities available-for-sale gross unrealized losses greater than one year	$ (270)	$ (738)
Securities available-for-sale agregate fair value of investment greater than one year	$ 9,687	$ 17,393
Number of securities held-to-maturity in one year or less \| security	51	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	$ 623	$ 1,453
Number of securities held-to-maturity greater than one year \| security	434	1,003
Securities held-to-maturity gross unrealized losses greater than one year	$ (25)	$ (277)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 2,898	$ 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	21	8
Securities available-for-sale gross unrealized losses one year or less	$ (43)	$ (12)
Securities available-for-sale aggregate fair value of investment one year or less	$ 4,500	$ 1,013
Number of Securities available for sale greater than one year \| security	29	41
Securities available-for-sale gross unrealized losses greater than one year	$ (216)	$ (353)
Securities available-for-sale agregate fair value of investment greater than one year	$ 5,809	$ 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	4	55
Securities available-for-sale gross unrealized losses one year or less	$ (2)	$ (6)
Securities available-for-sale aggregate fair value of investment one year or less	$ 202	$ 538
Number of Securities available for sale greater than one year \| security	86	159
Securities available-for-sale gross unrealized losses greater than one year	$ (11)	$ (185)
Securities available-for-sale agregate fair value of investment greater than one year	$ 1,129	$ 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	$ (1)	$ (8)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 413	$ 684
Number of securities held-to-maturity greater than one year \| security	119	511
Securities held-to-maturity gross unrealized losses greater than one year	$ (4)	$ (52)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 621	$ 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	19	8
Securities available-for-sale gross unrealized losses one year or less	$ (2)	$ (2)
Securities available-for-sale aggregate fair value of investment one year or less	$ 878	$ 106
Number of Securities available for sale greater than one year \| security	52	72
Securities available-for-sale gross unrealized losses greater than one year	$ (40)	$ (198)
Securities available-for-sale agregate fair value of investment greater than one year	$ 2,716	$ 4,825
Number of securities held-to-maturity in one year or less \| security	36	111
Securities held-to-maturity gross unrealized losses one year or less	$ 0	$ (14)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 210	$ 769
Number of securities held-to-maturity greater than one year \| security	314	491
Securities held-to-maturity gross unrealized losses greater than one year	$ (21)	$ (225)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 2,277	$ 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	$ (3)	$ (2)
Securities available-for-sale agregate fair value of investment greater than one year	$ 33	$ 42
Foreign Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	18	16
Securities available-for-sale gross unrealized losses one year or less	$ (1)	$ 0
Securities available-for-sale aggregate fair value of investment one year or less	$ 1,325	$ 1,724
Number of Securities available for sale greater than one year \| security	0	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	$ 0	$ 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	0	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, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Number of investments in debt securities 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, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

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