ACCUMULATED OTHER COMPREHENSIVE (LOSS) INCOME - Schedule of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pretax	$ (244)	$ (575)
Income Taxes	45	130
Net of Income Taxes	(199)	(445)
Unrealized loss on securities available for sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pretax	(440)	(762)
Income Taxes	95	178
Net of Income Taxes	(345)	(584)
Unrealized loss on securities available for sale transferred to held to maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pretax	(6)	(6)
Income Taxes	2	2
Net of Income Taxes	(4)	(4)
Defined benefit pension items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pretax	182	182
Income Taxes	(47)	(47)
Net of Income Taxes	135	135
Unrealized gain on cash flow hedge derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pretax	20	11
Income Taxes	(5)	(3)
Net of Income Taxes	$ 15	$ 8