Securities - Summaries of Fair Values and Unrealized Losses of Securities, Segregated by Length of Impairment (Detail) $ in Millions	Dec. 31, 2020 USD ($) Security	Dec. 31, 2019 USD ($) Security
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security	7	32
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security	17	78
Debt securities, available-for-sale, number of positions \| Security	24	110
Debt securities, available-for-sale, less than 12 months	$ 168	$ 685
Debt securities, available-for-sale, 12 months or longer	71	712
Debt securities, available-for-sale, unrealized loss position	239	1,397
Debt securities, available-for-sale, less than 12 months, accumulated loss	(1)	(4)
Debt securities, available-for-sale, 12 months or longer, accumulated loss	0	(6)
Debt securities, available-for-sale, accumulated loss	$ (1)	$ (10)
U.S. government agencies
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security	1	5
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security	16	15
Debt securities, available-for-sale, number of positions \| Security	17	20
Debt securities, available-for-sale, less than 12 months	$ 13	$ 48
Debt securities, available-for-sale, 12 months or longer	69	61
Debt securities, available-for-sale, unrealized loss position	82	109
Debt securities, available-for-sale, less than 12 months, accumulated loss	0	0
Debt securities, available-for-sale, 12 months or longer, accumulated loss	0	(1)
Debt securities, available-for-sale, accumulated loss	$ 0	$ (1)
U.S. government-sponsored entities
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security	1	0
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security	0	6
Debt securities, available-for-sale, number of positions \| Security	1	6
Debt securities, available-for-sale, less than 12 months	$ 25	$ 0
Debt securities, available-for-sale, 12 months or longer	0	130
Debt securities, available-for-sale, unrealized loss position	25	130
Debt securities, available-for-sale, less than 12 months, accumulated loss	0	0
Debt securities, available-for-sale, 12 months or longer, accumulated loss	0	0
Debt securities, available-for-sale, accumulated loss	$ 0	$ 0
Agency mortgage-backed securities
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security		13
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security		24
Debt securities, available-for-sale, number of positions \| Security		37
Debt securities, available-for-sale, less than 12 months		$ 200
Debt securities, available-for-sale, 12 months or longer		314
Debt securities, available-for-sale, unrealized loss position		514
Debt securities, available-for-sale, less than 12 months, accumulated loss		(1)
Debt securities, available-for-sale, 12 months or longer, accumulated loss		(2)
Debt securities, available-for-sale, accumulated loss		$ (3)
Agency collateralized mortgage obligations
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security	5	11
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security	0	32
Debt securities, available-for-sale, number of positions \| Security	5	43
Debt securities, available-for-sale, less than 12 months	$ 130	$ 323
Debt securities, available-for-sale, 12 months or longer	0	205
Debt securities, available-for-sale, unrealized loss position	130	528
Debt securities, available-for-sale, less than 12 months, accumulated loss	(1)	(1)
Debt securities, available-for-sale, 12 months or longer, accumulated loss	0	(3)
Debt securities, available-for-sale, accumulated loss	$ (1)	$ (4)
Non-agency collateralized mortgage obligations
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security		0
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security		0
Debt securities, available-for-sale, number of positions \| Security		0
Debt securities, available-for-sale, less than 12 months		$ 0
Debt securities, available-for-sale, 12 months or longer		0
Debt securities, available-for-sale, unrealized loss position		0
Debt securities, available-for-sale, less than 12 months, accumulated loss		0
Debt securities, available-for-sale, 12 months or longer, accumulated loss		0
Debt securities, available-for-sale, accumulated loss		$ 0
Commercial mortgage-backed securities
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security		3
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security		0
Debt securities, available-for-sale, number of positions \| Security		3
Debt securities, available-for-sale, less than 12 months		$ 114
Debt securities, available-for-sale, 12 months or longer		0
Debt securities, available-for-sale, unrealized loss position		114
Debt securities, available-for-sale, less than 12 months, accumulated loss		(2)
Debt securities, available-for-sale, 12 months or longer, accumulated loss		0
Debt securities, available-for-sale, accumulated loss		$ (2)
States of the U.S. and political subdivisions (municipals)
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security		0
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security		0
Debt securities, available-for-sale, number of positions \| Security		0
Debt securities, available-for-sale, less than 12 months		$ 0
Debt securities, available-for-sale, 12 months or longer		0
Debt securities, available-for-sale, unrealized loss position		0
Debt securities, available-for-sale, less than 12 months, accumulated loss		0
Debt securities, available-for-sale, 12 months or longer, accumulated loss		0
Debt securities, available-for-sale, accumulated loss		$ 0
Other debt securities
Schedule Of Securities [Line Items]
Debt securities, available-for-sale, less than 12 months, number of positions \| Security	0	0
Debt securities, available-for-sale, 12 months or longer, number of positions \| Security	1	1
Debt securities, available-for-sale, number of positions \| Security	1	1
Debt securities, available-for-sale, less than 12 months	$ 0	$ 0
Debt securities, available-for-sale, 12 months or longer	2	2
Debt securities, available-for-sale, unrealized loss position	2	2
Debt securities, available-for-sale, less than 12 months, accumulated loss	0	0
Debt securities, available-for-sale, 12 months or longer, accumulated loss	0	0
Debt securities, available-for-sale, accumulated loss	$ 0	$ 0

Schedule Of Securities [Line Items]

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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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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, 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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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 less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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