ACCUMULATED OTHER COMPREHENSIVE LOSS - Schedule of Accumulated Other Comprehensive Loss (Details) - USD ($)		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		$ 811,449,000	$ 690,506,000
Other comprehensive gain (loss) before reclassification		1,822,000	80,000
Reclassification to net income	[1]	638,000	634,000
TOTAL OTHER COMPREHENSIVE INCOME		2,460,000	714,000
BALANCE		846,697,000	716,201,000
Unrealized gain (loss) on available-for-sale debt securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		191,000	10,000
Other comprehensive gain (loss) before reclassification		1,855,000	47,000
TOTAL OTHER COMPREHENSIVE INCOME		1,855,000	47,000
BALANCE		2,046,000	57,000
Net unrealized gain (loss) on retirement plan [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE	[2]	(17,167,000)	(16,198,000)
Reclassification to net income	[1],[2]	638,000	634,000
TOTAL OTHER COMPREHENSIVE INCOME	[2]	638,000	634,000
BALANCE	[2]	(16,529,000)	(15,564,000)
Change in cumulative foreign currency translation adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		(21,000)	(46,000)
Other comprehensive gain (loss) before reclassification		(33,000)	33,000
TOTAL OTHER COMPREHENSIVE INCOME		(33,000)	33,000
BALANCE		(54,000)	(13,000)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		(16,997,000)	(16,234,000)
TOTAL OTHER COMPREHENSIVE INCOME		2,460,000	714,000
BALANCE		$ (14,537,000)	$ (15,520,000)

[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 $638,000 and $634,000 for the three months ended March 31, 2020 and 2019, respectively.

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