Equity Awards - Stock-based compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	$ 18,149	$ 14,610	$ 12,160
Tax benefit	4,809	3,771	3,126
Stock-based compensation expense, net of tax	13,340	10,839	9,034
Instructional and support costs
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	5,317	5,111	3,823
General and administration
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	13,535	9,499	7,970
Merger and integration costs
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	0	0	367
Restructuring costs
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	$ (703)	$ 0	$ 0