Significant Accounting Policies - Schedule of impact of Topic 606 adoption on Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Deferred costs, net	$ 65,896		$ 60,931
Liabilities:
Deferred revenue and student deposits	465,290		184,116
Deferred income taxes	257,083		247,371
Equity:
Accumulated deficit	(643,407)		$ (946,236)
Balances Without Adoption of ASC 606
Assets
Deferred costs, net	60,949
Liabilities:
Deferred revenue and student deposits	467,867
Deferred income taxes	257,047
Equity:
Accumulated deficit	(650,895)
ASU 2014-09 | Effect of Change Higher/(Lower)
Assets
Deferred costs, net	4,947
Liabilities:
Deferred revenue and student deposits	(2,577)
Deferred income taxes	36
Equity:
Accumulated deficit	$ 7,488	$ 1,400