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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, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Schedule of stock-based compensation expense        
Stock-based compensation expense included in operating expense $ 3,228 $ 2,036 $ 5,654 $ 4,926
Tax benefit 1,275 802 2,233 1,900
Stock-based compensation expense, net of income tax 1,953 1,234 3,421 3,026
Instruction and educational support [Member]        
Schedule of stock-based compensation expense        
Stock-based compensation expense included in operating expense 645 (438) 814 155
General and administration [Member]        
Schedule of stock-based compensation expense        
Stock-based compensation expense included in operating expense $ 2,583 $ 2,474 $ 4,840 $ 4,771