Significant Accounting Policies - Deferred Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Revenue Arrangement [Line Items]
Deferred revenue	$ 19,374	$ 20,639
Sales Revenue, Services, Net
Deferred Revenue Arrangement [Line Items]
Scholarship assistance	17,851	18,021	$ 7,583
Student deposits
Deferred Revenue Arrangement [Line Items]
Deferred revenue	$ 9,129	$ 9,158