ACCUMULATED OTHER COMPREHENSIVE LOSS - Schedule of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		$ 751,033	$ 650,681	$ 690,506	$ 659,054
Other comprehensive gain (loss) before reclassification				212	204
Reclassification to net income	[1]			1,903	1,266
TOTAL OTHER COMPREHENSIVE INCOME		686	457	2,115	1,470
BALANCE		786,567	675,507	786,567	675,507
Unrealized gain (loss) on available-for-sale debt securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE				10	(258)
Other comprehensive gain (loss) before reclassification				178	214
TOTAL OTHER COMPREHENSIVE INCOME				178	214
BALANCE		188	(44)	188	(44)
Net unrealized gain (loss) on retirement plan [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE	[2]			(16,198)	(11,169)
Reclassification to net income	[1],[2]			1,903	1,266
TOTAL OTHER COMPREHENSIVE INCOME	[2]			1,903	1,266
BALANCE	[2]	(14,295)	(9,903)	(14,295)	(9,903)
Change in cumulative foreign currency translation adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE				(46)	(37)
Other comprehensive gain (loss) before reclassification				34	(10)
TOTAL OTHER COMPREHENSIVE INCOME				34	(10)
BALANCE		(12)	(47)	(12)	(47)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		(14,805)	(10,451)	(16,234)	(11,464)
TOTAL OTHER COMPREHENSIVE INCOME		686	457	2,115	1,470
BALANCE		$ (14,119)	$ (9,994)	$ (14,119)	$ (9,994)

[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 $1.9 million and $1.3 million for the nine months ended September 30, 2019 and 2018, respectively.

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