ACCUMULATED OTHER COMPREHENSIVE LOSS - Schedule of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		$ 528,468	$ 466,765	$ 448,742
Other comprehensive loss before reclassification		(2,168)	(1,931)	(471)
Plan amendment cost				(5,963)
Reclassification to net income	[1]	1,370	1,084	997
TOTAL OTHER COMPREHENSIVE LOSS		(798)	(847)	(5,437)
BALANCE		659,054	528,468	466,765
Unrealized loss on available-for-sale securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		(246)	(111)	(28)
Other comprehensive loss before reclassification		(12)	(135)	(83)
TOTAL OTHER COMPREHENSIVE LOSS		(12)	(135)	(83)
BALANCE		(258)	(246)	(111)
Net unrealized gain (loss) on retirement plan [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE	[2]	(10,355)	(9,708)	(4,354)
Other comprehensive loss before reclassification	[2]	(2,184)	(1,731)	(388)
Plan amendment cost	[2]			(5,963)
Reclassification to net income	[1],[2]	1,370	1,084	997
TOTAL OTHER COMPREHENSIVE LOSS	[2]	(814)	(647)	(5,354)
BALANCE	[2]	(11,169)	(10,355)	(9,708)
Change in cumulative foreign currency translation adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		(65)
Other comprehensive loss before reclassification		28	(65)
TOTAL OTHER COMPREHENSIVE LOSS		28	(65)
BALANCE		(37)	(65)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
BALANCE		(10,666)	(9,819)	(4,382)
TOTAL OTHER COMPREHENSIVE LOSS		(798)	(847)	(5,437)
BALANCE		$ (11,464)	$ (10,666)	$ (9,819)

[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 $1.4 million, $1.1 million and $1.0 million for the years ended December 31, 2017, 2016 and 2015, respectively.

[2] Refer to Note 15: Employee Retirement Plans