CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
NET INCOME	$ 31,474	$ 5,959
OTHER COMPREHENSIVE INCOME, NET OF TAX:
Unrealized gain on available-for-sale securities, net of tax of $13 and $15	47	54
Amortization of prior service cost and actuarial loss for retirement plan included in net periodic pension costs, net of tax of $175 and $117, respectively	634	422
Change in cumulative foreign currency translation adjustment	33	20
TOTAL OTHER COMPREHENSIVE INCOME	714	496
COMPREHENSIVE INCOME	$ 32,188	$ 6,455