Summary of Significant Accounting Policies - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock-based compensation expense in operating income	$ 1,712	$ 2,084	$ 5,031	$ 5,524
Instructional costs and services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock-based compensation expense in operating income	404	418	1,213	1,206
Selling and promotional
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock-based compensation expense in operating income	207	173	600	435
General and administrative
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock-based compensation expense in operating income	$ 1,101	$ 1,493	$ 3,218	$ 3,883