Nature of Business and Significant Accounting Policies - Stock-based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total stock-based compensation expense	$ 5,912	$ 5,369	$ 4,024
Instructional costs and services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total stock-based compensation expense	1,598	1,274	876
Selling and promotional
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total stock-based compensation expense	684	568	444
General and administrative
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total stock-based compensation expense	$ 3,630	$ 3,527	$ 2,704