Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 24,058	$ 19,865
Other comprehensive income:
Foreign currency translation adjustments	2,929	2,468
Pension plan amortization of net actuarial loss and prior service cost, net of tax of $108 and $115 at respective dates	295	244
Comprehensive income	$ 27,282	$ 22,577