Equity Awards - Stock-based compensation (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	$ 3,900	$ 3,025
Tax benefit	1,000	777
Stock-based compensation expense, net of tax	2,900	2,248
Instructional and support costs
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	1,225	1,030
General and administration
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	$ 2,675	$ 1,995