Consolidated Statements of Comprehensive Income (Unaudited) - 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	$ 26,729	$ 17,296	$ 91,206	$ 62,191
Other comprehensive income/(loss):
Foreign currency translation adjustments	32	7,006	(10,504)	19,753
Pension plan amortization of net actuarial loss and prior service cost, net of tax of $69, $118, $246 and $350 at respective dates	202	250	703	737
Comprehensive income	$ 26,963	$ 24,552	$ 81,405	$ 82,681