OTHER COMPREHENSIVE INCOME (LOSS) Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Net unrealized gains on available-for-sale securities, beginning balance	$ 3,250	$ (17,826)
Net unrealized losses on cash flow hedging derivative instruments, beginning balance	(6,048)	(4,437)
Net unrecognized gains on postretirement plans, beginning balance	(3,470)	(2,157)
Accumulated other comprehensive (income) loss, beginning balance	(6,268)	(24,420)
Net Unrealized Gains (Losses) on AFS Securities
Other comprehensive income (loss) before reclassifications	18,930	10,899
Less: Amounts reclassified from AOCI	0	0
Other comprehensive income (loss)	18,930	10,899
Net Unrealized Losses on Cash Flow Hedges
Other comprehensive income (loss) before reclassifications	(902)	(925)
Less: Amounts reclassified from AOCI	(148)	(94)
Other comprehensive income (loss)	(754)	(831)
Defined Benefit Postretirement Plans
Other comprehensive income (loss) before reclassifications	138	53
Less: Amounts reclassified from AOCI	5	5
Other comprehensive income (loss)	133	48
Accumulated other comprehensive Income (Loss)
Other comprehensive income (loss) before reclassification	18,166	10,027
Less: Amounts reclassified from AOCI	(143)	(89)
Other Comprehensive Income (Loss)	18,309	10,116
Net unrealized gains on available-for-sale securities, ending balance	22,180	(6,927)
Net unrealized losses on cash flow hedging derivative instruments, ending balance	(6,802)	(5,268)
Net unrecognized gains on postretirement plans, ending balance	(3,337)	(2,109)
Accumulated other comprehensive (income) loss, ending balance	$ 12,041	$ (14,304)