CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 71,176	$ 193,468	$ 69,984
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments, net of $0 tax in 2020, 2019 and 2018	(183)	(518)	(236)
Unrealized gain on marketable securities, net of $0 tax in 2020, 2019 and 2018	307	849	161
Unrealized actuarial gain (loss) on pension benefits, net of tax of ($308), $497, and ($144) in 2020, 2019 and 2018, respectively	843	(1,772)	525
Total other comprehensive income (loss)	967	(1,441)	450
TOTAL COMPREHENSIVE INCOME	$ 72,143	$ 192,027	$ 70,434