Income Taxes - Deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of tax effects of principal temporary differences that give rise to deferred tax assets
Stock-based compensation	$ 15,024	$ 20,477
Property and equipment	(3,326)	(6,059)
Other facility-related costs	2,622	5,242
Deferred revenue	5,445	7,182
Tuition receivable		3,922
Deferred leasing costs	1,789	3,064
Prepaid compensation	(373)	(1,139)
Other	3,271	(1,592)
Net deferred tax asset	$ 24,452	$ 31,097