Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	$ 12,944	$ 13,287
Minimum protective endowment held in an interest-bearing account	500	500
U.S. Higher Education
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	1,800	1,400
Australia / New Zealand
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	11,100	$ 11,900
PENNSYLVANIA
Restricted Cash and Cash Equivalents Items [Line Items]
Minimum protective endowment held in an interest-bearing account	$ 500