Stock-Based Compensation - Summary of Cost Charged Against Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	$ 1,566	$ 1,255	$ 4,262	$ 3,972
Tax benefit	(620)	(504)	(1,688)	(1,566)
Stock-based compensation expense, net of tax	946	751	2,574	2,406
Instructional costs and services
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	229	274	958	1,077
Selling and promotional
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	200	168	559	524
General and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	$ 1,137	$ 813	$ 2,745	$ 2,371