Restructuring Charges - Changes to Restructuring Liability by Major Restructuring Activity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Restructuring Cost And Reserve [Line Items]
Beginning balance	$ 2,549
Restructuring charges	162	$ 394
Cash payments	(482)
Adjustment to liability	(14)
Ending balance	2,215
GES | Severance & Employee Benefits
Restructuring Cost And Reserve [Line Items]
Beginning balance	1,551
Restructuring charges	32
Cash payments	(167)
Adjustment to liability	451
Ending balance	1,867
GES | Facilities
Restructuring Cost And Reserve [Line Items]
Beginning balance	807
Cash payments	(20)
Adjustment to liability	(451)
Ending balance	336
Other Restructuring | Severance & Employee Benefits
Restructuring Cost And Reserve [Line Items]
Beginning balance	191
Restructuring charges	130
Cash payments	(295)
Adjustment to liability	(14)
Ending balance	$ 12