Equity Awards - Stock-based compensation (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	$ 3,010	$ 2,688
Tax benefit	759	752
Stock-based compensation expense, net of tax	2,251	1,936
Instructional and support costs
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	858	620
General and administration
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	1,673	2,068
Merger and integration costs
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	$ 479	$ 0