Other Assets - Schedule of Other Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses, net of current portion	$ 20,606		$ 22,418
Equity method investments	14,532	$ 17,401	15,795	$ 16,734	$ 16,990	$ 15,795
Cloud computing arrangements	6,665		6,385
Other investments	2,999		2,527
Tuition receivable, non-current	2,912		3,585
Other	6,710		4,218
Other assets	$ 54,424		$ 54,928