OTHER COMPREHENSIVE INCOME (LOSS) Components of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AFS Securities:
Unrealized holding gains, pre-tax amount	$ (22,305)	$ 24,115
Unrealized holdings gains, tax (expense) benefit	4,796	(5,185)
Unrealized holdings gains, after-tax amount	(17,509)	18,930
Net unrealized gains, pre-tax amount	(22,305)	24,115
Net unrealized gains, tax (expense) benefit	4,796	(5,185)
Net unrealized gains, after-tax amount	(17,509)	18,930
Cash Flow Hedges:
Net decrease in fair value, pre-tax amount	6,392	(1,149)
Net decrease in fair value, tax (expense) benefit	(1,374)	247
Net decrease in fair value, after-tax amount	5,018	(902)
Less: effective portion reclassified into interest expense, after-tax amount	(146)	(148)
Net decrease in fair value, pre-tax amount	6,578	(961)
Net decrease in fair value, tax (expense) benefit	(1,414)	207
Net decrease in fair value, after-tax amount	5,164	(754)
Postretirement Plans:
Net actuarial gain, after-tax amount	175	138
Net gain on postretirement plans, pre-tax amount	218	169
Net gain on postretirement plans, tax (expense) benefit	(48)	(36)
Net gain on postretirement plans, net of tax	170	133
Other comprehensive income, pre-tax amount	(15,509)	23,323
Other comprehensive income, tax (expense) benefit	3,334	(5,014)
Other comprehensive (loss) income	(12,175)	18,309
Interest Expense, Borrowings
Cash Flow Hedges:
Less: effective portion reclassified into interest expense, pre-tax amount	(578)	(240)
Less: effective portion reclassified into interest expense, tax (expense) benefit	124	52
Less: effective portion reclassified into interest expense, after-tax amount	(454)	(188)
Interest Income, Interest and Fees on Loans
Cash Flow Hedges:
Less: effective portion reclassified into interest expense, pre-tax amount	392	52
Less: effective portion reclassified into interest expense, tax (expense) benefit	(84)	(12)
Less: effective portion reclassified into interest expense, after-tax amount	$ 308	$ 40