ACCUMULATED OTHER COMPREHENSIVE LOSS - Schedule of Accumulated Other Comprehensive Loss (Details) - USD ($)		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		$ 912,714,000	$ 811,449,000
Other comprehensive gain (loss) before reclassification		(164,000)	1,822,000
Reclassification to net income	[1]	1,174,000	638,000
TOTAL OTHER COMPREHENSIVE INCOME		1,010,000	2,460,000
BALANCE		952,468,000	846,697,000
Unrealized gain (loss) on available-for-sale securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		91,000	191,000
Other comprehensive gain (loss) before reclassification		(58,000)	1,855,000
TOTAL OTHER COMPREHENSIVE INCOME		(58,000)	1,855,000
BALANCE		33,000	2,046,000
Net unrealized gain (loss) on retirement plan [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE	[2]	(36,075,000)	(17,167,000)
Reclassification to net income	[1],[2]	1,174,000	638,000
TOTAL OTHER COMPREHENSIVE INCOME	[2]	1,174,000	638,000
BALANCE	[2]	(34,901,000)	(16,529,000)
Change in cumulative foreign currency translation adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		(35,000)	(21,000)
Other comprehensive gain (loss) before reclassification		(106,000)	(33,000)
TOTAL OTHER COMPREHENSIVE INCOME		(106,000)	(33,000)
BALANCE		(141,000)	(54,000)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		(36,019,000)	(16,997,000)
TOTAL OTHER COMPREHENSIVE INCOME		1,010,000	2,460,000
BALANCE		$ (35,009,000)	$ (14,537,000)

[1] The Company reclassified amortization of prior service cost and actuarial loss for its retirement plan from accumulated other comprehensive loss to net income of $1.2 million and $638,000 for the three months ended March 31, 2021 and 2020.

[2] Refer to Note 14: Supplemental Executive Retirement Plan