Business Combinations - Revenue and Net Loss (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
Business Acquisition [Line Items]
Revenues	$ 267,494	$ 239,026	$ 255,831	$ 265,302	$ 263,772	$ 241,747	$ 245,110	$ 246,508	$ 1,027,653	$ 997,137	$ 634,185
Net income (loss)	$ 5,917	$ 10,960	$ 34,152	$ 35,239	$ 28,537	$ 16,692	$ 24,409	$ 11,500	86,268	$ 81,138	(15,671)
ANZ
Business Acquisition [Line Items]
Revenues									23,400
Net income (loss)									$ (10,500)
Capella Education Company
Business Acquisition [Line Items]
Revenues											160,400
Net income (loss)											$ (39,600)