Restructuring Charges - Changes to Restructuring Liability by Major Restructuring Activity (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Restructuring Cost And Reserve [Line Items]
Beginning balance			$ 2,251
Restructuring charges	$ 4,455	$ 662	5,143	$ 824
Cash payments			(4,132)
Adjustment to liability			27
Ending balance	3,289		3,289
GES | Severance & Employee Benefits
Restructuring Cost And Reserve [Line Items]
Beginning balance			2,039
Restructuring charges			3,916
Cash payments			(3,900)
Adjustment to liability			(33)
Ending balance	2,022		2,022
GES | Facilities
Restructuring Cost And Reserve [Line Items]
Beginning balance			200
Restructuring charges			1,084
Cash payments			(174)
Adjustment to liability			53
Ending balance	1,163		1,163
Other Restructuring | Severance & Employee Benefits
Restructuring Cost And Reserve [Line Items]
Beginning balance			12
Restructuring charges			143
Cash payments			(58)
Adjustment to liability			7
Ending balance	$ 104		$ 104