CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
NET INCOME	$ 17,667	$ 16,506	$ 47,248	$ 44,507
Other comprehensive income, net of tax:
Foreign currency translation adjustments, net of $0 tax in each of the three and nine months ended September 30, 2018 and 2017	(184)	2	(184)	97
Unrealized gain on marketable securities, net of $0 tax in each of the three and nine months ended September 30, 2018 and 2017	165	47	153	145
Amortization of defined benefit pension items, net of tax of $9 and $27 in the three and nine months ended September 30, 2018, respectively, and $14 and $41 in the three and nine months ended September 30, 2017, respectively	31	50	94	148
Total other comprehensive income	12	99	63	390
TOTAL COMPREHENSIVE INCOME	$ 17,679	$ 16,605	$ 47,311	$ 44,897