CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
NET INCOME	$ 133,372	$ 138,304	$ 58,840
OTHER COMPREHENSIVE (LOSS) INCOME, NET OF TAX:
Unrealized (loss) gain on available-for-sale securities, net of tax of $28, $51 and $74, respectively	(100)	181	268
Employee benefit plan:
Actuarial loss on retirement plan, net of tax of $3,569, $988 and $1,841, respectively	(21,464)	(3,492)	(6,690)
Amortization of prior service cost and actuarial loss for retirement plan included in net periodic pension costs, net of tax of $723, $713 and $457, respectively	2,556	2,523	1,661
Net change in employee benefit plan	(18,908)	(969)	(5,029)
Change in cumulative foreign currency translation adjustment	(14)	25	(9)
TOTAL OTHER COMPREHENSIVE LOSS	(19,022)	(763)	(4,770)
COMPREHENSIVE INCOME	$ 114,350	$ 137,541	$ 54,070