ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS) (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Attributable to Parent [Roll Forward]
OCI Before Reclassifications							$ (3,000)	$ (1,300)
Amounts Reclassified from AOCI							7,500	6,000
Net OCI	$ 2,477	$ 881	$ 1,188	$ 2,161	$ 553	$ 1,948	4,546	4,662
Total
AOCI Attributable to Parent [Roll Forward]
Beginning balance			(76,400)			(65,100)	(76,400)	(65,100)
Net OCI	2,477	$ 881	1,188	2,161	$ 553	1,948
Ending balance	(71,900)			(60,400)			(71,900)	(60,400)
Qualified Cash Flow Hedging Instruments
AOCI Attributable to Parent [Roll Forward]
Beginning balance			(1,400)			(3,000)	(1,400)	(3,000)
OCI Before Reclassifications							0	0
Amounts Reclassified from AOCI							1,000	1,200
Net OCI							1,000	1,200
Ending balance	(400)			(1,800)			(400)	(1,800)
Unrealized Gains/(Losses) on Marketable Securities
AOCI Attributable to Parent [Roll Forward]
Beginning balance			1,100			700	1,100	700
OCI Before Reclassifications							(600)	300
Amounts Reclassified from AOCI							0	0
Net OCI							(600)	300
Ending balance	500			1,000			500	1,000
Defined Benefit Plans
AOCI Attributable to Parent [Roll Forward]
Beginning balance			$ (76,100)			$ (62,800)	(76,100)	(62,800)
OCI Before Reclassifications							(2,400)	(1,600)
Amounts Reclassified from AOCI							6,500	4,800
Net OCI							4,100	3,200
Ending balance	$ (72,000)			$ (59,600)			$ (72,000)	$ (59,600)