Restructuring Charges - Changes to Restructuring Liability by Major Restructuring Activity (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Restructuring Cost And Reserve [Line Items]
Beginning balance			$ 3,782
Restructuring charges	$ 255	$ 1,697	817	$ 3,664
Cash payments			(1,948)
Adjustment to liability			2
Ending balance	2,653		2,653
GES | Severance & Employee Benefits
Restructuring Cost And Reserve [Line Items]
Beginning balance			2,274
Restructuring charges			442
Cash payments			(1,048)
Ending balance	1,668		1,668
GES | Facilities
Restructuring Cost And Reserve [Line Items]
Beginning balance			1,092
Restructuring charges			237
Cash payments			(449)
Ending balance	880		880
Other Restructuring | Severance & Employee Benefits
Restructuring Cost And Reserve [Line Items]
Beginning balance			416
Restructuring charges			138
Cash payments			(451)
Adjustment to liability			2
Ending balance	$ 105		$ 105