CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
NET INCOME	$ 58,840	$ 103,885	$ 48,070
OTHER COMPREHENSIVE INCOME (LOSS), NET OF TAX:
Unrealized gain (loss) on available-for-sale securities, net of tax of $74, $7 and $72, respectively	268	(12)	(135)
Employee benefit plan:
Actuarial loss on retirement plan, net of tax of $1,841, $1,047 and $945, respectively	(6,690)	(1,904)	(1,731)
Plan amendment cost, net of tax of none, $154 and none, respectively		(280)
Amortization of plan amendment cost, prior service cost and actuarial loss for retirement plan included in net periodic pension costs, net of tax of $457, $754 and $591, respectively	1,661	1,370	1,084
Net change for employee benefit plan	(5,029)	(814)	(647)
Change in cumulative foreign currency translation adjustment	(9)	28	(65)
TOTAL OTHER COMPREHENSIVE LOSS	(4,770)	(798)	(847)
COMPREHENSIVE INCOME	$ 54,070	$ 103,087	$ 47,223