Revenues - Deferred Income Activity (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Deferred Revenue and Contract Balances
Beginning balance	$ 20,237	$ 11,226	$ 13,280	$ 13,544
Increases to deferred income	10,786	1,681	22,457	4,429
Recognition of revenue	(5,802)	(1,287)	(10,516)	(6,353)
Ending balance	25,221	11,620	25,221	11,620
Cumulative catch-up adjustment to revenue	1,300
Advisory services
Deferred Revenue and Contract Balances
Recognition of revenue	(554)	(554)	(1,700)	(2,000)
Audio visual
Deferred Revenue and Contract Balances
Recognition of revenue	(458)	(194)	(1,600)	(2,600)
Other revenue
Deferred Revenue and Contract Balances
Recognition of revenue	(4,000)		(1,900)
Other services
Deferred Revenue and Contract Balances
Recognition of revenue	$ (773)	$ (539)	$ (5,400)	$ (1,800)