Stock-Based Compensation - Summary of Cost Charged Against Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	$ 1,450	$ 1,179	$ 2,696	$ 2,681
Tax benefit	(574)	(467)	(1,068)	(1,062)
Stock-based compensation expense, net of tax	876	712	1,628	1,619
Instructional costs and services
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	417	373	729	803
Selling and promotional
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	184	172	359	356
General and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	$ 849	$ 634	$ 1,608	$ 1,522