ACCUMULATED OTHER COMPREHENSIVE LOSS - Schedule of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		$ 811,449	$ 690,506	$ 659,054
Other comprehensive gain (loss) before reclassification		(21,578)	(3,311)	(6,431)
Reclassification to net income	[1]	2,556	2,548	1,661
TOTAL OTHER COMPREHENSIVE LOSS		(19,022)	(763)	(4,770)
BALANCE		912,714	811,449	690,506
Unrealized gain (loss) on available-for-sale securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		191	10	(258)
Other comprehensive gain (loss) before reclassification		(100)	181	268
TOTAL OTHER COMPREHENSIVE LOSS		(100)	181	268
BALANCE		91	191	10
Net unrealized gain (loss) on retirement plan [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE	[2]	(17,167)	(16,198)	(11,169)
Other comprehensive gain (loss) before reclassification	[2]	(21,464)	(3,492)	(6,690)
Reclassification to net income	[1],[2]	2,556	2,523	1,661
TOTAL OTHER COMPREHENSIVE LOSS	[2]	(18,908)	(969)	(5,029)
BALANCE	[2]	(36,075)	(17,167)	(16,198)
Change in cumulative foreign currency translation adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		(21)	(46)	(37)
Other comprehensive gain (loss) before reclassification		(14)		(9)
Reclassification to net income	[1]		25
TOTAL OTHER COMPREHENSIVE LOSS		(14)	25	(9)
BALANCE		(35)	(21)	(46)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		(16,997)	(16,234)	(11,464)
TOTAL OTHER COMPREHENSIVE LOSS		(19,022)	(763)	(4,770)
BALANCE		$ (36,019)	$ (16,997)	$ (16,234)

[1] The Company reclassified amortization of prior service cost, actuarial loss and plan amendment cost for its retirement plan from accumulated other comprehensive loss to net income of $2.6 million, $2.5 million and $1.7 million for the years ended December 31, 2020, 2019 and 2018, respectively.

[2] Refer to Note 15: Employee Retirement Plans