CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 27,609	$ 48,898	$ 39,152
Other comprehensive income:
Foreign currency translation adjustments, net of $0 tax in 2017, 2016 and 2015	79	(384)	(191)
Unrealized loss on marketable securities, net of $0 tax in 2017, 2016 and 2015	(207)	(123)	(180)
Unrealized actuarial gain (loss) on pension benefits, net of tax of ($194), $98 and $96 in 2017, 2016 and 2015, respectively	699	(352)	(344)
Total other comprehensive income (loss)	571	(859)	(715)
Total comprehensive income	28,180	48,039	38,437
Foreign currency translation adjustment, tax	0	0	0
Unrealized gain on marketable securities, tax	0	0	0
Unrealized actuarial gain (loss) on pension benefits, tax	$ (194)	$ 98	$ 96