OTHER COMPREHENSIVE INCOME (LOSS) Components of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
AFS Debt Securities:
Unrealized holding gains, pre-tax amount	$ (92,753)	$ (22,305)
Unrealized holdings gains, tax (expense) benefit	19,942	4,796
Unrealized holdings gains, after-tax amount	(72,811)	(17,509)
Net unrealized gains, pre-tax amount	(92,753)	(22,305)
Net unrealized gains, tax (expense) benefit	19,942	4,796
Net unrealized gains, after-tax amount	(72,811)	(17,509)
Cash Flow Hedges:
Net decrease in fair value, pre-tax amount	3,635	6,392
Net decrease in fair value, tax (expense) benefit	(782)	(1,374)
Net decrease in fair value, after-tax amount	2,853	5,018
Net decrease in fair value, pre-tax amount	3,753	6,578
Net decrease in fair value, tax (expense) benefit	(808)	(1,414)
Net decrease in fair value, after-tax amount	2,945	5,164
Postretirement Plans:
Net gain on postretirement plans, pre-tax amount	232	218
Net gain on postretirement plans, tax (expense) benefit	(50)	(48)
Net gain on postretirement plans, net of tax	182	170
Other comprehensive income, pre-tax amount	(88,768)	(15,509)
Other comprehensive income, tax (expense) benefit	19,084	3,334
Other comprehensive loss	(69,684)	(12,175)
Interest Expense, Borrowings
Cash Flow Hedges:
Less: effective portion reclassified into interest expense, pre-tax amount	(500)	(578)
Less: effective portion reclassified into interest expense, tax (expense) benefit	108	124
Less: effective portion reclassified into interest expense, after-tax amount	(392)	(454)
Interest Income, Interest and Fees on Loans
Cash Flow Hedges:
Less: effective portion reclassified into interest expense, pre-tax amount	382	392
Less: effective portion reclassified into interest expense, tax (expense) benefit	(82)	(84)
Less: effective portion reclassified into interest expense, after-tax amount	$ 300	$ 308