CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
NET INCOME	$ 14,200	$ 14,099
Other comprehensive income, net of tax:
Foreign currency translation adjustments, net of $0 tax in each of the three months ended March 31, 2018 and 2017	(37)	64
Unrealized gain (loss) on marketable securities, net of $0 tax in each of the three months ended March 31, 2018 and 2017	(238)	83
Amortization of defined benefit pension items, net of tax of $9 and $13 in the three months ended March 31, 2018 and 2017, respectively	32	49
Total other comprehensive income (loss)	(243)	196
TOTAL COMPREHENSIVE INCOME	$ 13,957	$ 14,295