Other Comprehensive Loss - Related Tax Effects Allocated to Each Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Equity [Abstract]
Cumulative translation adjustment, before tax amount	$ (5,304)	$ 2,256	$ (3,834)	$ 4,078
Amortization of actuarial losses, before tax amount	(763)	(761)	(1,270)	(1,718)
Total other comprehensive income (loss), before tax amount	(6,067)	1,495	(5,104)	2,360
Cumulative translation adjustment, tax	0	0	0	0
Amortization of actuarial losses, tax	170	217	338	434
Total other comprehensive income (loss), tax	170	217	338	434
Cumulative translation adjustment, after tax amount	(5,304)	2,256	(3,834)	4,078
Amortization of actuarial losses, after tax amount	(593)	(544)	(932)	(1,284)
Total other comprehensive income (loss)	$ (5,897)	$ 1,712	$ (4,766)	$ 2,794