CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
NET INCOME	$ 103,885	$ 48,070	$ 14,678
OTHER COMPREHENSIVE LOSS, NET OF TAX:
Unrealized loss on available-for-sale securities, net of tax of $7, $72 and $46, respectively	(12)	(135)	(83)
Employee benefit plan:
Actuarial loss on retirement plan, net of tax of $1,047, $945 and $218, respectively	(1,904)	(1,731)	(388)
Plan amendment cost, net of tax of $154, none and $3,305, respectively	(280)		(5,963)
Amortization of plan amendment cost, prior service cost and actuarial loss for retirement plan included in net periodic pension costs, net of tax of $754, $591 and $553, respectively	1,370	1,084	997
Net change for employee benefit plan	(814)	(647)	(5,354)
Change in cumulative foreign currency translation adjustment	28	(65)
TOTAL OTHER COMPREHENSIVE LOSS	(798)	(847)	(5,437)
COMPREHENSIVE INCOME	$ 103,087	$ 47,223	$ 9,241