Significant Accounting Policies - Tuition Receivable (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Schedule of tuition receivable and allowance for doubtful accounts
Tuition receivable	$ 33,183		$ 30,733
Allowance for doubtful accounts	(12,557)	$ (11,760)	(10,201)	$ (9,794)	$ (9,981)	$ (10,024)
Tuition receivable, net	20,626		20,532
Other Assets [Member]
Schedule of tuition receivable and allowance for doubtful accounts
Tuition receivable included in other assets	$ 2,600		$ 2,300