Equity Awards - Stock-based compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	$ 14,610	$ 12,160	$ 15,532
Tax benefit	3,771	3,126	3,922
Stock-based compensation expense, net of tax	10,839	9,034	11,610
Instructional and support costs
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	5,111	3,823	2,588
General and administration
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	9,499	7,970	10,702
Merger and integration costs
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	$ 0	$ 367	$ 2,242