Summary of Significant Accounting Policies - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock-based compensation expense in operating income	$ 1,750	$ 1,689
Instructional costs and services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock-based compensation expense in operating income	478	403
Selling and promotional
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock-based compensation expense in operating income	258	194
General and administrative
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock-based compensation expense in operating income	$ 1,014	$ 1,092