Restructuring Charges - Changes to Restructuring Liability by Major Restructuring Activity (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restructuring Cost And Reserve [Line Items]
Beginning balance					$ 4,513				$ 2,251	$ 4,513	$ 2,251	$ 2,549
Restructuring charges		$ 1,070	$ 11,259	$ 260	851	$ 1,535	$ 1,702	$ 4,455	688	13,440	8,380	1,587
Cash payments										(10,825)	(6,193)	(1,716)
Non-cash items	[1]									(1,789)
Adjustment to liability										(109)	75	(169)
Ending balance		5,230				4,513				5,230	4,513	2,251
GES | Severance & Employee Benefits
Restructuring Cost And Reserve [Line Items]
Beginning balance					2,935				2,039	2,935	2,039	1,551
Restructuring charges										6,563	6,071	1,457
Cash payments										(7,051)	(5,169)	(1,379)
Adjustment to liability										(7)	(6)	410
Ending balance		2,440				2,935				2,440	2,935	2,039
GES | Facilities
Restructuring Cost And Reserve [Line Items]
Beginning balance					1,339				200	1,339	200	807
Restructuring charges										5,784	1,817
Cash payments										(2,573)	(752)	(156)
Non-cash items	[1]									(1,789)
Adjustment to liability										5	74	(451)
Ending balance		2,766				1,339				2,766	1,339	200
Other Restructuring | Severance & Employee Benefits
Restructuring Cost And Reserve [Line Items]
Beginning balance					$ 239				$ 12	239	12	191
Restructuring charges										1,093	492	130
Cash payments										(1,201)	(272)	(181)
Adjustment to liability										(107)	7	(128)
Ending balance		$ 24				$ 239				$ 24	$ 239	$ 12

[1] Represents the impact related to the closure and liquidation of GES’ United Kingdom-based audio-visual services business during the year ended December 31, 2020.