Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Dec. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	42	78
Securities available-for-sale gross unrealized losses one year or less	$ (29)	$ (73)
Securities available-for-sale aggregate fair value of investment one year or less	$ 3,818	$ 8,005
Number of Securities available for sale greater than one year \| security	31	171
Securities available-for-sale gross unrealized losses greater than one year	$ (43)	$ (257)
Securities available-for-sale agregate fair value of investment greater than one year	$ 1,566	$ 8,467
Number of securities held-to-maturity in one year or less \| security		62
Securities held-to-maturity gross unrealized losses one year or less		$ (3)
Securities held-to-maturity aggregate fair value of investment one year or less		$ 950
Number of securities held-to-maturity greater than one year \| security		351
Securities held-to-maturity gross unrealized losses greater than one year		$ (24)
Securities held-to-maturity aggregate fair value of investment greater than one year		$ 2,227
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	5	22
Securities available-for-sale gross unrealized losses one year or less	$ (5)	$ (61)
Securities available-for-sale aggregate fair value of investment one year or less	$ 751	$ 4,034
Number of Securities available for sale greater than one year \| security	13	28
Securities available-for-sale gross unrealized losses greater than one year	$ (42)	$ (208)
Securities available-for-sale agregate fair value of investment greater than one year	$ 1,271	$ 4,962
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	15	18
Securities available-for-sale gross unrealized losses one year or less	$ (3)	$ (8)
Securities available-for-sale aggregate fair value of investment one year or less	$ 715	$ 772
Number of Securities available for sale greater than one year \| security	8	83
Securities available-for-sale gross unrealized losses greater than one year	$ (1)	$ (6)
Securities available-for-sale agregate fair value of investment greater than one year	$ 46	$ 672
Number of securities held-to-maturity in one year or less \| security		15
Securities held-to-maturity gross unrealized losses one year or less		$ 0
Securities held-to-maturity aggregate fair value of investment one year or less		$ 63
Number of securities held-to-maturity greater than one year \| security		76
Securities held-to-maturity gross unrealized losses greater than one year		$ (5)
Securities held-to-maturity aggregate fair value of investment greater than one year		$ 522
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	13	27
Securities available-for-sale gross unrealized losses one year or less	$ (11)	$ (4)
Securities available-for-sale aggregate fair value of investment one year or less	$ 1,482	$ 1,961
Number of Securities available for sale greater than one year \| security	3	50
Securities available-for-sale gross unrealized losses greater than one year	$ 0	$ (41)
Securities available-for-sale agregate fair value of investment greater than one year	$ 8	$ 2,508
Number of securities held-to-maturity in one year or less \| security		46
Securities held-to-maturity gross unrealized losses one year or less		$ (3)
Securities held-to-maturity aggregate fair value of investment one year or less		$ 887
Number of securities held-to-maturity greater than one year \| security		274
Securities held-to-maturity gross unrealized losses greater than one year		$ (19)
Securities held-to-maturity aggregate fair value of investment greater than one year		$ 1,705
Asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	3	0
Securities available-for-sale gross unrealized losses one year or less	$ (10)	$ 0
Securities available-for-sale aggregate fair value of investment one year or less	$ 104	$ 0
Number of Securities available for sale greater than one year \| security	2	5
Securities available-for-sale gross unrealized losses greater than one year	$ 0	$ (2)
Securities available-for-sale agregate fair value of investment greater than one year	$ 0	$ 33
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	6	11
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	$ 766	$ 1,238
Number of Securities available for sale greater than one year \| security	5	5
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	$ 241	$ 292
Obligations of U.S. states and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities held-to-maturity in one year or less \| security		1
Securities held-to-maturity gross unrealized losses one year or less		$ 0
Securities held-to-maturity aggregate fair value of investment one year or less		$ 0
Number of securities held-to-maturity greater than one year \| security		1
Securities held-to-maturity gross unrealized losses greater than one year		$ 0
Securities held-to-maturity aggregate fair value of investment greater than one year		$ 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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