CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Comprehensive Income Net Of Tax Including Portion Attributable To Noncontrolling Interest [Abstract]
Net income	$ 14,778	$ 11,534	$ 30,144	$ 16,445
Unrealized (loss) on defined benefit plan, net of tax	(1,135)	(9,571)	(2,517)	(14,732)
Unrealized gain on interest rate swap, net of tax	180		60
Unrealized foreign currency gain (loss), net of tax	8,249	1,187	25,416	(6,588)
Comprehensive income (loss)	22,072	3,150	53,103	(4,875)
Less: Comprehensive income attributable to noncontrolling interest	(328)	(886)	(1,298)	(1,778)
Total comprehensive income attributable to common stockholders	$ 21,744	$ 2,264	$ 51,805	$ (6,653)