Equity Awards - Stock-based compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	$ 21,792	$ 18,149	$ 14,610
Tax benefit	5,488	4,809	3,771
Stock-based compensation expense, net of tax	16,304	13,340	10,839
Instructional and support costs
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	7,026	5,317	5,111
General and administration
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	14,766	13,535	9,499
Restructuring costs
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	$ 0	$ (703)	$ 0