CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
NET INCOME	$ 10,814	$ 47,187	$ 16,773	$ 57,552
OTHER COMPREHENSIVE INCOME (LOSS), NET OF TAX:
Unrealized gain on available-for-sale securities, net of tax of $32, $58, $48 and $50, respectively	117	105	171	90
Employee benefit plan:
Actuarial loss, net of tax of none and none, respectively		(1,662)		(1,662)
Amortization of prior service cost and actuarial loss for retirement plan included in net periodic pension costs, net of tax of $117, $187, $234 and $350, respectively	422	343	844	642
Net change for employee benefit plan	422	(1,319)	844	(1,020)
Change in cumulative foreign currency translation adjustment	(22)	(5)	(2)	25
TOTAL OTHER COMPREHENSIVE INCOME (LOSS)	517	(1,219)	1,013	(905)
COMPREHENSIVE INCOME	$ 11,331	$ 45,968	$ 17,786	$ 56,647