Restructuring Charges - Changes to Restructuring Liability by Major Restructuring Activity (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Cost And Reserve [Line Items]
Beginning balance				$ 3,782				$ 2,276	$ 3,782	$ 2,276	$ 1,944
Restructuring charges	$ 187	$ 255	$ 168	394	$ 1,519	$ 1,697	$ 975	992	1,004	5,183	2,956
Cash payments									(2,253)	(3,866)	(2,572)
Adjustment to liability									16	189	(52)
Ending balance	2,549				3,782				2,549	3,782	2,276
GES Consolidation | Severance & Employee Benefits
Restructuring Cost And Reserve [Line Items]
Beginning balance				2,274				751	2,274	751	543
Restructuring charges									442	3,693	1,767
Cash payments									(1,165)	(2,170)	(1,514)
Adjustment to liability											(45)
Ending balance	1,551				2,274				1,551	2,274	751
GES Consolidation | Facilities
Restructuring Cost And Reserve [Line Items]
Beginning balance				1,092				1,291	1,092	1,291	1,161
Restructuring charges									265	759	587
Cash payments									(550)	(1,150)	(457)
Adjustment to liability										192
Ending balance	807				1,092				807	1,092	1,291
Other Restructuring | Severance & Employee Benefits
Restructuring Cost And Reserve [Line Items]
Beginning balance				$ 416				$ 234	416	234	240
Restructuring charges									297	731	602
Cash payments									(538)	(546)	(601)
Adjustment to liability									16	(3)	(7)
Ending balance	$ 191				$ 416				$ 191	$ 416	$ 234