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Stock Options, Restricted Stock and Restricted Stock Units - Stock-based compensation (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Schedule of stock-based compensation expense        
Stock-based compensation expense included in operating expense $ 3,250 $ 3,228 $ 5,937 $ 5,654
Tax benefit 894 1,275 1,646 2,233
Stock-based compensation expense, net of income tax 2,356 1,953 4,291 3,421
Instruction and educational support [Member]        
Schedule of stock-based compensation expense        
Stock-based compensation expense included in operating expense 499 645 1,118 814
General and administration [Member]        
Schedule of stock-based compensation expense        
Stock-based compensation expense included in operating expense $ 2,751 $ 2,583 $ 4,819 $ 4,840