Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Sep. 30, 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	120	42
Securities available-for-sale gross unrealized losses one year or less	$ (231)	$ (29)
Securities available-for-sale aggregate fair value of investment one year or less	$ 16,077	$ 3,818
Number of Securities available for sale greater than one year \| security	37	31
Securities available-for-sale gross unrealized losses greater than one year	$ (62)	$ (43)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 1,558	$ 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	17	5
Securities available-for-sale gross unrealized losses one year or less	$ (48)	$ (5)
Securities available-for-sale aggregate fair value of investment one year or less	$ 2,416	$ 751
Number of Securities available for sale greater than one year \| security	9	13
Securities available-for-sale gross unrealized losses greater than one year	$ (42)	$ (42)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 752	$ 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	60	15
Securities available-for-sale gross unrealized losses one year or less	$ (127)	$ (3)
Securities available-for-sale aggregate fair value of investment one year or less	$ 5,879	$ 715
Number of Securities available for sale greater than one year \| security	11	8
Securities available-for-sale gross unrealized losses greater than one year	$ (4)	$ (1)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 185	$ 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	38	13
Securities available-for-sale gross unrealized losses one year or less	$ (56)	$ (11)
Securities available-for-sale aggregate fair value of investment one year or less	$ 7,274	$ 1,482
Number of Securities available for sale greater than one year \| security	11	3
Securities available-for-sale gross unrealized losses greater than one year	$ (9)	$ 0
Securities available-for-sale aggregate fair value of investment greater than one year	$ 495	$ 8
Asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	0	3
Securities available-for-sale gross unrealized losses one year or less	$ 0	$ (10)
Securities available-for-sale aggregate fair value of investment one year or less	$ 0	$ 104
Number of Securities available for sale greater than one year \| security	5	2
Securities available-for-sale gross unrealized losses greater than one year	$ (7)	$ 0
Securities available-for-sale aggregate fair value of investment greater than one year	$ 101	$ 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	$ 508	$ 766
Number of Securities available for sale greater than one year \| security	1	5
Securities available-for-sale gross unrealized losses greater than one year	$ 0	$ 0
Securities available-for-sale aggregate fair value of investment greater than one year	$ 25	$ 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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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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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