Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	$ 9,794	$ 14,011
Restricted cash included in other assets	500	500
Cash and cash equivalents	268,918	187,509
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	279,212	202,020	$ 420,497	$ 312,237
U.S. Higher Education
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	700	100
Australia / New Zealand
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	$ 9,100	$ 13,900