SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated other comprehensive income (loss)
Beginning Balance	$ 677,055
Adjust funded status of pension plan, net of tax of $0.0, $0.7 and $0.7 million for the years December 31, 2017, 2016 and 2015 respectively	1,357	$ 5,251	$ (809)
Foreign currency translation adjustment	1,265	(687)	26
Reclassifications of gains on sale of marketable securities to net income	(1,044)
Unrealized gain on securities	440	868
Ending Balance	688,727	677,055
Adjust funded status of pension plan, tax	700	700	700
Accumulated Other Comprehensive Income/(Loss)
Accumulated other comprehensive income (loss)
Beginning Balance	1,728	(3,704)	(2,921)
Adjust funded status of pension plan, net of tax of $0.0, $0.7 and $0.7 million for the years December 31, 2017, 2016 and 2015 respectively	1,357	5,251	(809)
Foreign currency translation adjustment	1,265	(687)	26
Reclassifications of gains on sale of marketable securities to net income	(1,044)
Unrealized gain on securities	440	868
Ending Balance	3,746	1,728	(3,704)
Pension and other postretirement benefit plans
Accumulated other comprehensive income (loss)
Beginning Balance	1,770	(3,481)	(2,672)
Adjust funded status of pension plan, net of tax of $0.0, $0.7 and $0.7 million for the years December 31, 2017, 2016 and 2015 respectively	1,357	5,251	(809)
Ending Balance	3,127	1,770	(3,481)
Translation Adjustment
Accumulated other comprehensive income (loss)
Beginning Balance	(910)	(223)	(249)
Foreign currency translation adjustment	1,265	(687)	26
Ending Balance	355	(910)	$ (223)
Investment
Accumulated other comprehensive income (loss)
Beginning Balance	868
Reclassifications of gains on sale of marketable securities to net income	(1,044)
Unrealized gain on securities	440	868
Ending Balance	$ 264	$ 868