OTHER COMPREHENSIVE INCOME (LOSS) Components of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AFS Securities:
Unrealized holding gains, pre-tax amount	$ 24,115	$ 13,884
Unrealized holdings gains, tax (expense) benefit	(5,185)	(2,985)
Unrealized holdings gains, after-tax amount	18,930	10,899
Net unrealized gains, pre-tax amount	24,115	13,884
Net unrealized gains, tax (expense) benefit	(5,185)	(2,985)
Net unrealized gains, after-tax amount	18,930	10,899
Cash Flow Hedges:
Net decrease in fair value, pre-tax amount	(1,149)	(1,178)
Net decrease in fair value, tax (expense) benefit	247	253
Net decrease in fair value, after-tax amount	(902)	(925)
Less: effective portion reclassified into interest expense, pre-tax amount		(119)
Less: effective portion reclassified into interest expense, tax (expense) benefit		25
Less: effective portion reclassified into interest expense, after-tax amount	(148)	(94)
Net decrease in fair value, pre-tax amount	(961)	(1,059)
Net decrease in fair value, tax (expense) benefit	207	228
Net decrease in fair value, after-tax amount	(754)	(831)
Postretirement Plans:
Net actuarial gain, pre-tax amount	175	67
Net actuarial gain, tax (expense) benefit	(37)	(14)
Net actuarial gain, after-tax amount	138	53
Less: Amortiaztion of net prior service credits, pre-tax amount	6	6
Less: Amortiaztion of net prior service credits, tax (expense) benefit	(1)	(1)
Less: Amortiaztion of net prior service credits, after-tax amount	5	5
Net gain on postretirement plans, pre-tax amount	169	61
Net gain on postretirement plans, tax (expense) benefit	(36)	(13)
Net gain on postretirement plans, net of tax	133	48
Other comprehensive income, pre-tax amount	23,323	12,886
Other comprehensive income, tax (expense) benefit	(5,014)	(2,770)
Other comprehensive income	18,309	$ 10,116
Interest Expense, Borrowings
Cash Flow Hedges:
Less: effective portion reclassified into interest expense, pre-tax amount	(240)
Less: effective portion reclassified into interest expense, tax (expense) benefit	52
Less: effective portion reclassified into interest expense, after-tax amount	(188)
Interest Income, Interest and Fees on Loans
Cash Flow Hedges:
Less: effective portion reclassified into interest expense, pre-tax amount	52
Less: effective portion reclassified into interest expense, tax (expense) benefit	(12)
Less: effective portion reclassified into interest expense, after-tax amount	$ 40