Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Mar. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	101	42
Securities available-for-sale gross unrealized losses one year or less	$ (296)	$ (29)
Securities available-for-sale aggregate fair value of investment one year or less	$ 11,123	$ 3,818
Number of Securities available for sale greater than one year \| security	29	31
Securities available-for-sale gross unrealized losses greater than one year	$ (54)	$ (43)
Securities available-for-sale agregate fair value of investment greater than one year	$ 1,068	$ 1,566
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	18	5
Securities available-for-sale gross unrealized losses one year or less	$ (96)	$ (5)
Securities available-for-sale aggregate fair value of investment one year or less	$ 2,882	$ 751
Number of Securities available for sale greater than one year \| security	11	13
Securities available-for-sale gross unrealized losses greater than one year	$ (50)	$ (42)
Securities available-for-sale agregate fair value of investment greater than one year	$ 755	$ 1,271
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	58	15
Securities available-for-sale gross unrealized losses one year or less	$ (150)	$ (3)
Securities available-for-sale aggregate fair value of investment one year or less	$ 5,120	$ 715
Number of Securities available for sale greater than one year \| security	8	8
Securities available-for-sale gross unrealized losses greater than one year	$ (1)	$ (1)
Securities available-for-sale agregate fair value of investment greater than one year	$ 45	$ 46
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	18	13
Securities available-for-sale gross unrealized losses one year or less	$ (49)	$ (11)
Securities available-for-sale aggregate fair value of investment one year or less	$ 2,648	$ 1,482
Number of Securities available for sale greater than one year \| security	4	3
Securities available-for-sale gross unrealized losses greater than one year	$ (1)	$ 0
Securities available-for-sale agregate fair value of investment greater than one year	$ 50	$ 8
Asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	2	3
Securities available-for-sale gross unrealized losses one year or less	$ (1)	$ (10)
Securities available-for-sale aggregate fair value of investment one year or less	$ 48	$ 104
Number of Securities available for sale greater than one year \| security	3	2
Securities available-for-sale gross unrealized losses greater than one year	$ (2)	$ 0
Securities available-for-sale agregate fair value of investment greater than one year	$ 57	$ 0
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	5	6
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	$ 425	$ 766
Number of Securities available for sale greater than one year \| security	3	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	$ 161	$ 241

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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