Other Comprehensive Loss - Related Tax Effects Allocated to Each Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Equity [Abstract]
Cumulative translation adjustment, before tax amount	$ 1,470	$ 1,822
Amortization of actuarial losses, before tax amount	(506)	(957)
Total other comprehensive income (loss), before tax amount	964	865
Cumulative translation adjustment, tax	0	0
Amortization of actuarial losses, tax	168	217
Total other comprehensive income (loss), tax	168	217
Cumulative translation adjustment, after tax amount	1,470	1,822
Amortization of actuarial losses, after tax amount	(338)	(740)
Total other comprehensive income (loss)	$ 1,132	$ 1,082