ACCUMULATED OTHER COMPREHENSIVE (LOSS) INCOME - Schedule of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pretax	$ (35)	$ (575)
Income Taxes	(9)	130
Net of Income Taxes	(44)	(445)
Unrealized loss on securities available for sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pretax	(224)	(762)
Income Taxes	40	178
Net of Income Taxes	(184)	(584)
Unrealized loss on securities available for sale transferred to held to maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pretax	(5)	(6)
Income Taxes	1	2
Net of Income Taxes	(4)	(4)
Defined benefit pension items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pretax	177	182
Income Taxes	(46)	(47)
Net of Income Taxes	131	135
Unrealized gain on cash flow hedge derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pretax	17	11
Income Taxes	(4)	(3)
Net of Income Taxes	$ 13	$ 8