CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
NET INCOME	$ 22,818	$ 13,520	$ 39,591	$ 71,072
OTHER COMPREHENSIVE INCOME (LOSS), NET OF TAX:
Unrealized gain on available-for-sale securities, net of tax of $12, $14, $60 and $64, respectively	43	27	214	117
Employee benefit plan:
Actuarial loss, net of tax of none, $154, none and $1,060, respectively		(283)		(1,945)
Amortization of prior service cost and actuarial loss for retirement plan included in net periodic pension costs, net of tax of $117, $199, $351 and $549, respectively	422	365	1,266	1,007
Net change for employee benefit plan	422	82	1,266	(938)
Change in cumulative foreign currency translation adjustment	(8)	3	(10)	28
TOTAL OTHER COMPREHENSIVE INCOME (LOSS)	457	112	1,470	(793)
COMPREHENSIVE INCOME	$ 23,275	$ 13,632	$ 41,061	$ 70,279