CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
NET INCOME	$ 17,099	$ 17,667	$ 35,177	$ 47,248
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments, net of $0 tax in each of the three and nine months ended September 30, 2019 and 2018	27	(184)	(245)	(184)
Unrealized gain on marketable securities, net of $0 tax in each of the three and nine months ended September 30, 2019 and 2018	208	165	799	153
Amortization of defined benefit pension items, net of tax of $5 and $15 in the three and nine months ended September 30, 2019, respectively, and $9 and $27 in the three and nine months ended September 30, 2018, respectively	17	31	51	94
Total other comprehensive income	252	12	605	63
TOTAL COMPREHENSIVE INCOME	$ 17,351	$ 17,679	$ 35,782	$ 47,311