Significant Accounting Policies - Tuition Receivable (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of tuition receivable and allowance for doubtful accounts
Tuition receivable	$ 35,809	$ 30,733
Allowance for doubtful accounts	(12,687)	(10,201)	$ (10,024)	$ (8,835)
Tuition receivable, net	23,122	20,532
Other Assets [Member]
Schedule of tuition receivable and allowance for doubtful accounts
Tuition receivable, noncurrent	$ 2,900	$ 2,300