Stock-Based Compensation - Summary of Stock-based Compensation Cost Charged Against Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	$ 1,340	$ 1,267	$ 4,083	$ 3,691
Tax benefit	(546)	(502)	(1,669)	(1,461)
Stock-based compensation expense, net of tax	794	765	2,414	2,230
Instructional costs and services
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	378	312	1,156	920
Selling and promotional
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	165	131	493	392
General and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	$ 797	$ 824	$ 2,434	$ 2,379