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Stock Options, Restricted Stock and Restricted Stock Units - Stock-based compensation (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Schedule of stock-based compensation expense    
Stock-based compensation expense included in operating expense $ 2,427 $ 2,891
Tax benefit 958 1,099
Stock-based compensation expense, net of income tax 1,469 1,792
Instruction and educational support [Member]    
Schedule of stock-based compensation expense    
Stock-based compensation expense included in operating expense 170 651
General and administration [Member]    
Schedule of stock-based compensation expense    
Stock-based compensation expense included in operating expense $ 2,257 $ 2,240