Significant Accounting Policies - Tuition Receivable (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Schedule of tuition receivable and allowance for doubtful accounts
Tuition receivable	$ 38,772	$ 35,809
Allowance for doubtful accounts	(13,775)	(12,687)	$ (10,820)	$ (10,201)
Tuition receivable, net	24,997	23,122
Other assets
Schedule of tuition receivable and allowance for doubtful accounts
Tuition receivable, noncurrent	$ 3,300	$ 2,900