Overview and Summary of Significant Accounting Policies - Schedule of Corrections to Prior Period Adjustments (Details) - USD ($) $ in Thousands	3 Months Ended							12 Months Ended
	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
Total revenue								$ 415,435	$ 1,302,736	$ 1,237,324
Services
Total revenue								351,528	1,101,534	1,051,389
Costs and expenses								457,827	1,031,187	980,543
Cost of Services
Costs and expenses									1,031,187	980,543
GES
Total revenue								$ 338,625	1,079,923	1,052,037
Previously Reported
Total revenue	$ 63,072	$ 30,863	$ 306,008	$ 321,334	$ 353,743	$ 402,279	$ 285,594
Previously Reported | Services
Total revenue									1,170,493	1,110,249
Previously Reported | Cost of Services
Costs and expenses									1,100,146	1,039,403
Revision Of Prior Period Error Correction Adjustment | GES | North America | Services
Total revenue									(51,927)	(43,603)
Revision Of Prior Period Error Correction Adjustment | GES | North America | Cost of Services
Costs and expenses									(51,927)	(43,603)
Revision Of Prior Period Error Correction Adjustment | GES | EMEA | Services
Total revenue									(17,032)	(15,257)
Revision Of Prior Period Error Correction Adjustment | GES | EMEA | Cost of Services
Costs and expenses									$ (17,032)	$ (15,257)