OTHER COMPREHENSIVE INCOME (LOSS) Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Net unrealized gains on available-for-sale securities, beginning balance	$ 29,310	$ 3,250
Net unrealized losses on cash flow hedging derivative instruments, beginning balance	(4,626)	(6,048)
Net unrecognized gains on postretirement plans, beginning balance	(3,944)	(3,470)
Accumulated other comprehensive (income) loss, beginning balance	20,740	(6,268)
Net Unrealized Gains (Losses) on AFS Securities
Other comprehensive income (loss) before reclassifications	(17,509)	18,930
Less: Amounts reclassified from AOCI	0	0
Other comprehensive income (loss)	(17,509)	18,930
Net Unrealized Losses on Cash Flow Hedges
Other comprehensive income (loss) before reclassifications	5,018	(902)
Less: Amounts reclassified from AOCI	(146)	(148)
Other comprehensive income (loss)	5,164	(754)
Defined Benefit Postretirement Plans
Other comprehensive income (loss) before reclassifications	175	138
Less: Amounts reclassified from AOCI	5	5
Other comprehensive income (loss)	170	133
Accumulated other comprehensive Income (Loss)
Other comprehensive income (loss) before reclassification	(12,316)	18,166
Less: Amounts reclassified from AOCI	(141)	(143)
Other Comprehensive Income (Loss)	(12,175)	18,309
Net unrealized gains on available-for-sale securities, ending balance	11,801	22,180
Net unrealized losses on cash flow hedging derivative instruments, ending balance	538	(6,802)
Net unrecognized gains on postretirement plans, ending balance	(3,774)	(3,337)
Accumulated other comprehensive (income) loss, ending balance	$ 8,565	$ 12,041