ACCUMULATED OTHER COMPREHENSIVE (LOSS) INCOME - Schedule of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pretax	$ (761)	$ (637)
Income Taxes	179	146
Net of Income Taxes	(582)	(491)
Unrealized loss on securities available for sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pretax	(876)	(752)
Income Taxes	208	175
Net of Income Taxes	(668)	(577)
Unrealized loss on securities available for sale transferred to held to maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pretax	(7)	(7)
Income Taxes	2	2
Net of Income Taxes	(5)	(5)
Defined benefit pension items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pretax	122	122
Income Taxes	(31)	(31)
Net of Income Taxes	$ 91	$ 91