CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 80,459	$ 48,283	$ 46,320
Foreign currency translation adjustments	28,463	(5,081)	(27,172)
Derivative instruments designated as cash flow hedges:
Unrealized gain (loss) on derivative instruments held at year end (net of taxes of $15 in 2017, $0 in 2016, $5 in 2015)	26	0	(2)
Unrealized gain reclassified into earnings	0	11	84
Accumulated other comprehensive income (loss) pension and post-retirement benefits:
Actuarial net gain (loss) incurred in fiscal year, net of tax (Note 4)	(1,481)	1,106	2,760
Amortization of gain, net of tax (Note 4)	99	160	966
Other comprehensive income (loss)	27,107	(3,804)	(23,364)
Comprehensive income	$ 107,566	$ 44,479	$ 22,956