ACCUMULATED OTHER COMPREHENSIVE LOSS - Schedule of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		$ 659,054	$ 528,468	$ 466,765
Other comprehensive gain (loss) before reclassification		(6,431)	(2,168)	(1,931)
Reclassification to net income	[1]	1,661	1,370	1,084
TOTAL OTHER COMPREHENSIVE LOSS		(4,770)	(798)	(847)
BALANCE		690,506	659,054	528,468
Unrealized gain (loss) on available-for-sale securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		(258)	(246)	(111)
Other comprehensive gain (loss) before reclassification		268	(12)	(135)
TOTAL OTHER COMPREHENSIVE LOSS		268	(12)	(135)
BALANCE		10	(258)	(246)
Net unrealized gain (loss) on retirement plan [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE	[2]	(11,169)	(10,355)	(9,708)
Other comprehensive gain (loss) before reclassification	[2]	(6,690)	(2,184)	(1,731)
Reclassification to net income	[1],[2]	1,661	1,370	1,084
TOTAL OTHER COMPREHENSIVE LOSS	[2]	(5,029)	(814)	(647)
BALANCE	[2]	(16,198)	(11,169)	(10,355)
Change in cumulative foreign currency translation adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		(37)	(65)
Other comprehensive gain (loss) before reclassification		(9)	28	(65)
TOTAL OTHER COMPREHENSIVE LOSS		(9)	28	(65)
BALANCE		(46)	(37)	(65)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		(11,464)	(10,666)	(9,819)
TOTAL OTHER COMPREHENSIVE LOSS		(4,770)	(798)	(847)
BALANCE		$ (16,234)	$ (11,464)	$ (10,666)

[1] The Company reclassified amortization of plan amendment cost, prior service cost, and actuarial loss for its retirement plan from accumulated other comprehensive loss to net income of $1.7 million, $1.4 million and $1.1 million for the years ended December 31, 2018, 2017 and 2016, respectively.

[2] Refer to Note 15: Employee Retirement Plans