SECURITIES - Fair Values and Unrealized Losses of Impaired Securities, by Length of Impairment (Detail) $ in Millions	Sep. 30, 2020 USD ($) Security	Dec. 31, 2019 USD ($) Security
Schedule Of Securities [Line Items]
Number of temporarily impaired available for sale securities, Less than 1 year \| Security	4	32
Number of temporarily impaired available for sale securities, Greater than 1 year \| Security	18	78
Number of temporarily impaired available for sale securities \| Security	22	110
Temporarily impaired securities available for sale, less than 12 months, fair value	$ 118	$ 685
Temporarily impaired securities available for sale, 12 months or more, fair value	76	712
Temporarily impaired securities available for sale, fair value, total	194	1,397
Temporarily impaired securities available for sale, less than 12 months, unrealized losses	0	(4)
Temporarily impaired securities available for sale, 12 months or more, unrealized losses	(1)	(6)
Temporarily impaired securities available for sale, unrealized losses, total	$ (1)	$ (10)
U.S. Treasury [Member]
Schedule Of Securities [Line Items]
Number of temporarily impaired available for sale securities, Less than 1 year \| Security	1
Number of temporarily impaired available for sale securities, Greater than 1 year \| Security	0
Number of temporarily impaired available for sale securities \| Security	1
Temporarily impaired securities available for sale, less than 12 months, fair value	$ 100
Temporarily impaired securities available for sale, 12 months or more, fair value	0
Temporarily impaired securities available for sale, fair value, total	100
Temporarily impaired securities available for sale, less than 12 months, unrealized losses	0
Temporarily impaired securities available for sale, 12 months or more, unrealized losses	0
Temporarily impaired securities available for sale, unrealized losses, total	$ 0
US Government Agencies [Member]
Schedule Of Securities [Line Items]
Number of temporarily impaired available for sale securities, Less than 1 year \| Security	1	5
Number of temporarily impaired available for sale securities, Greater than 1 year \| Security	17	15
Number of temporarily impaired available for sale securities \| Security	18	20
Temporarily impaired securities available for sale, less than 12 months, fair value	$ 13	$ 48
Temporarily impaired securities available for sale, 12 months or more, fair value	74	61
Temporarily impaired securities available for sale, fair value, total	87	109
Temporarily impaired securities available for sale, less than 12 months, unrealized losses	0	0
Temporarily impaired securities available for sale, 12 months or more, unrealized losses	(1)	(1)
Temporarily impaired securities available for sale, unrealized losses, total	$ (1)	$ (1)
U.S. Government-Sponsored Entities [Member]
Schedule Of Securities [Line Items]
Number of temporarily impaired available for sale securities, Less than 1 year \| Security		0
Number of temporarily impaired available for sale securities, Greater than 1 year \| Security		6
Number of temporarily impaired available for sale securities \| Security		6
Temporarily impaired securities available for sale, less than 12 months, fair value		$ 0
Temporarily impaired securities available for sale, 12 months or more, fair value		130
Temporarily impaired securities available for sale, fair value, total		130
Temporarily impaired securities available for sale, less than 12 months, unrealized losses		0
Temporarily impaired securities available for sale, 12 months or more, unrealized losses		0
Temporarily impaired securities available for sale, unrealized losses, total		$ 0
Agency Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Number of temporarily impaired available for sale securities, Less than 1 year \| Security		13
Number of temporarily impaired available for sale securities, Greater than 1 year \| Security		24
Number of temporarily impaired available for sale securities \| Security		37
Temporarily impaired securities available for sale, less than 12 months, fair value		$ 200
Temporarily impaired securities available for sale, 12 months or more, fair value		314
Temporarily impaired securities available for sale, fair value, total		514
Temporarily impaired securities available for sale, less than 12 months, unrealized losses		(1)
Temporarily impaired securities available for sale, 12 months or more, unrealized losses		(2)
Temporarily impaired securities available for sale, unrealized losses, total		$ (3)
Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Number of temporarily impaired available for sale securities, Less than 1 year \| Security	2	11
Number of temporarily impaired available for sale securities, Greater than 1 year \| Security	0	32
Number of temporarily impaired available for sale securities \| Security	2	43
Temporarily impaired securities available for sale, less than 12 months, fair value	$ 5	$ 323
Temporarily impaired securities available for sale, 12 months or more, fair value	0	205
Temporarily impaired securities available for sale, fair value, total	5	528
Temporarily impaired securities available for sale, less than 12 months, unrealized losses	0	(1)
Temporarily impaired securities available for sale, 12 months or more, unrealized losses	0	(3)
Temporarily impaired securities available for sale, unrealized losses, total	$ 0	$ (4)
Commercial Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Number of temporarily impaired available for sale securities, Less than 1 year \| Security		3
Number of temporarily impaired available for sale securities, Greater than 1 year \| Security		0
Number of temporarily impaired available for sale securities \| Security		3
Temporarily impaired securities available for sale, less than 12 months, fair value		$ 114
Temporarily impaired securities available for sale, 12 months or more, fair value		0
Temporarily impaired securities available for sale, fair value, total		114
Temporarily impaired securities available for sale, less than 12 months, unrealized losses		(2)
Temporarily impaired securities available for sale, 12 months or more, unrealized losses		0
Temporarily impaired securities available for sale, unrealized losses, total		$ (2)
Other Debt Securities [Member]
Schedule Of Securities [Line Items]
Number of temporarily impaired available for sale securities, Less than 1 year \| Security	0	0
Number of temporarily impaired available for sale securities, Greater than 1 year \| Security	1	1
Number of temporarily impaired available for sale securities \| Security	1	1
Temporarily impaired securities available for sale, less than 12 months, fair value	$ 0	$ 0
Temporarily impaired securities available for sale, 12 months or more, fair value	2	2
Temporarily impaired securities available for sale, fair value, total	2	2
Temporarily impaired securities available for sale, less than 12 months, unrealized losses	0	0
Temporarily impaired securities available for sale, 12 months or more, unrealized losses	0	0
Temporarily impaired securities available for sale, unrealized losses, total	$ 0	$ 0

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