Significant Accounting Policies - Tuition Receivable (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Tuition receivable	$ 112,648	$ 99,942
Allowance for credit losses	(49,179)	(49,773)
Tuition receivable, net	63,469	50,169
Other Assets
Accounts, Notes, Loans and Financing Receivable [Line Items]
Tuition receivable, noncurrent	$ 3,400	$ 3,600