CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
NET INCOME	$ 36,962	$ 22,818	$ 111,876	$ 39,591
OTHER COMPREHENSIVE INCOME, NET OF TAX:
Unrealized gain on available-for-sale securities, net of tax of $7, $12, $49 and $60, respectively	27	43	178	214
Amortization of prior service cost and actuarial loss for retirement plan included in net periodic pension costs, net of tax of $175, $117, $525 and $351, respectively	634	422	1,903	1,266
Change in cumulative foreign currency translation adjustment	25	(8)	34	(10)
TOTAL OTHER COMPREHENSIVE INCOME	686	457	2,115	1,470
COMPREHENSIVE INCOME	$ 37,648	$ 23,275	$ 113,991	$ 41,061