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Stock Options, Restricted Stock and Restricted Stock Units - Stock-based compensation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Schedule of stock-based compensation expense      
Stock-based compensation expense included in operating expense $ 10,767 $ 10,213 $ 9,453
Tax benefit 4,256 4,032 3,734
Stock-based compensation expense, net of income tax 6,511 6,181 5,719
Instruction and educational support [Member]      
Schedule of stock-based compensation expense      
Stock-based compensation expense included in operating expense 1,432 2,134 1,736
Marketing [Member]      
Schedule of stock-based compensation expense      
Stock-based compensation expense included in operating expense
Admissions advisory [Member]      
Schedule of stock-based compensation expense      
Stock-based compensation expense included in operating expense
General and administration [Member]      
Schedule of stock-based compensation expense      
Stock-based compensation expense included in operating expense $ 9,335 $ 8,079 $ 7,717