SECURITIES - Fair Values and Unrealized Losses of Impaired Securities, by Length of Impairment (Detail) $ in Millions	Jun. 30, 2021 USD ($) Security	Dec. 31, 2020 USD ($) Security
Schedule Of Securities [Line Items]
Number of temporarily impaired available for sale securities, Less than 1 year \| Security	24	7
Number of temporarily impaired available for sale securities, Greater than 1 year \| Security	12	17
Number of temporarily impaired available for sale securities, total \| Security	36	24
Temporarily impaired securities available for sale, less than 12 months, fair value	$ 766	$ 168
Temporarily impaired securities available for sale, 12 months or more, fair value	24	71
Temporarily impaired securities available for sale, fair value, total	790	239
Temporarily impaired securities available for sale, less than 12 months, unrealized losses	(7)	(1)
Temporarily impaired securities available for sale, 12 months or more, unrealized losses	0	0
Temporarily impaired securities available for sale, unrealized losses, total	$ (7)	$ (1)
US Government Agencies [Member]
Schedule Of Securities [Line Items]
Number of temporarily impaired available for sale securities, Less than 1 year \| Security	1	1
Number of temporarily impaired available for sale securities, Greater than 1 year \| Security	11	16
Number of temporarily impaired available for sale securities, total \| Security	12	17
Temporarily impaired securities available for sale, less than 12 months, fair value	$ 0	$ 13
Temporarily impaired securities available for sale, 12 months or more, fair value	22	69
Temporarily impaired securities available for sale, fair value, total	22	82
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
U.S. Government-Sponsored Entities [Member]
Schedule Of Securities [Line Items]
Number of temporarily impaired available for sale securities, Less than 1 year \| Security	2	1
Number of temporarily impaired available for sale securities, Greater than 1 year \| Security	0	0
Number of temporarily impaired available for sale securities, total \| Security	2	1
Temporarily impaired securities available for sale, less than 12 months, fair value	$ 74	$ 25
Temporarily impaired securities available for sale, 12 months or more, fair value	0	0
Temporarily impaired securities available for sale, fair value, total	74	25
Temporarily impaired securities available for sale, less than 12 months, unrealized losses	(1)	0
Temporarily impaired securities available for sale, 12 months or more, unrealized losses	0	0
Temporarily impaired securities available for sale, unrealized losses, total	$ (1)	$ 0
Agency 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, total \| Security	3
Temporarily impaired securities available for sale, less than 12 months, fair value	$ 152
Temporarily impaired securities available for sale, 12 months or more, fair value	0
Temporarily impaired securities available for sale, fair value, total	152
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 Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Number of temporarily impaired available for sale securities, Less than 1 year \| Security	12	5
Number of temporarily impaired available for sale securities, Greater than 1 year \| Security	0	0
Number of temporarily impaired available for sale securities, total \| Security	12	5
Temporarily impaired securities available for sale, less than 12 months, fair value	$ 493	$ 130
Temporarily impaired securities available for sale, 12 months or more, fair value	0	0
Temporarily impaired securities available for sale, fair value, total	493	130
Temporarily impaired securities available for sale, less than 12 months, unrealized losses	(5)	(1)
Temporarily impaired securities available for sale, 12 months or more, unrealized losses	0	0
Temporarily impaired securities available for sale, unrealized losses, total	$ (5)	$ (1)
Commercial Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Number of temporarily impaired available for sale securities, Less than 1 year \| Security	2
Number of temporarily impaired available for sale securities, Greater than 1 year \| Security	0
Number of temporarily impaired available for sale securities, total \| Security	2
Temporarily impaired securities available for sale, less than 12 months, fair value	$ 37
Temporarily impaired securities available for sale, 12 months or more, fair value	0
Temporarily impaired securities available for sale, fair value, total	37
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	0
Temporarily impaired securities available for sale, unrealized losses, total	$ (1)
States of the U.S. and Political Subdivisions (municipals) [Member]
Schedule Of Securities [Line Items]
Number of temporarily impaired available for sale securities, Less than 1 year \| Security	4
Number of temporarily impaired available for sale securities, Greater than 1 year \| Security	0
Number of temporarily impaired available for sale securities, total \| Security	4
Temporarily impaired securities available for sale, less than 12 months, fair value	$ 10
Temporarily impaired securities available for sale, 12 months or more, fair value	0
Temporarily impaired securities available for sale, fair value, total	10
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
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, total \| 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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