Stock-Based Compensation - Summary of Cost Charged Against Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	$ 1,843	$ 1,246
Tax benefit	(490)	(494)
Stock-based compensation expense, net of tax	1,353	752
Instructional costs and services
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	377	312
Selling and promotional
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	240	175
General and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	$ 1,226	$ 759