ACCUMULATED OTHER COMPREHENSIVE (LOSS) INCOME - Schedule of Pretax and After-tax Components of Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI activity before reclassification, net amount	$ (91)	$ 53
Realized loss on sale of investment securities available for sale, net	0	14
Provision for investment securities available for sale credit losses	0	4
Amounts reclassified from AOCI to earnings, net amount	0	13
Other comprehensive income (loss) on securities available for sale, gross amount	(124)	88
Other comprehensive loss on securities available for sale, tax	33	(22)
Other comprehensive (loss) income, net of tax	(91)	66
Unrealized (loss) gain on securities available for sale:
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI activity before reclassification, gross amount	(124)	60
AOCI activity before reclassification, tax	33	(15)
AOCI activity before reclassification, net amount	(91)	45
Other comprehensive income (loss) on securities available for sale, gross amount	(124)	78
Other comprehensive loss on securities available for sale, tax	33	(20)
Other comprehensive (loss) income, net of tax	(91)	58
Unrealized (loss) gain on securities available for sale: \| Realized loss on sale of investment securities available for sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from AOCI to earnings, gross amount	0	18
Amounts reclassified from AOCI to earnings, tax	0	(5)
Amounts reclassified from AOCI to earnings, net amount	0	13
Defined benefit pension items
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI activity before reclassification, gross amount	0	10
AOCI activity before reclassification, tax	0	(2)
AOCI activity before reclassification, net amount	0	8
Amounts reclassified from AOCI to earnings, gross amount	0	0
Amounts reclassified from AOCI to earnings, tax	0	0
Amounts reclassified from AOCI to earnings, net amount	0	0
Other comprehensive income (loss) on securities available for sale, gross amount	0	10
Other comprehensive loss on securities available for sale, tax	0	(2)
Other comprehensive (loss) income, net of tax	$ 0	$ 8

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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