Summary of Significant Accounting Policies - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock-based compensation expense in operating income	$ 1,573	$ 1,630	$ 3,323	$ 3,319
Instructional costs and services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock-based compensation expense in operating income	402	406	880	809
Selling and promotional
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock-based compensation expense in operating income	218	199	476	393
General and administrative
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock-based compensation expense in operating income	$ 953	$ 1,025	$ 1,967	$ 2,117