CONSOLIDATED STATEMENTS OF COMPREHENSIVE (LOSS) INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Comprehensive Income Net Of Tax Including Portion Attributable To Noncontrolling Interest [Abstract]
Net income	$ 266	$ 18,477	$ 26,513
Unrealized (loss) gain on defined benefit plan, net of tax	(4,897)	(7,777)	4,399
Unrealized gain on interest rate swap, net of tax	1,018	1,506
Unrealized foreign currency gain (loss), net of tax	33,065	(21,979)	(20,413)
Comprehensive income (loss)	29,452	(9,773)	10,499
Less: Comprehensive income attributable to noncontrolling interest	(2,071)	(2,542)	(2,239)
Total comprehensive income (loss) attributable to common stockholders	$ 27,381	$ (12,315)	$ 8,260