Stock Options, Restricted Stock and Restricted Stock Units - Stock-based compensation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	$ 2,914	$ 2,404	$ 8,569	$ 7,330
Tax benefit	1,151	957	3,384	2,857
Stock-based compensation expense, net of income tax	1,763	1,447	5,185	4,473
Instruction and educational support [Member]
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	548	522	1,363	678
General and administration [Member]
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	$ 2,366	$ 1,882	$ 7,206	$ 6,652