v3.26.1
Summary of significant accounting policies - Narrative (Details) - USD ($)
 3 Months Ended 6 Months Ended
Dec. 22, 2017
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2028
Jul. 01, 2027
Dec. 31, 2025
Summary of significant accounting policies                
Collateral account in connection with company's corporate card program   $ 2,800,000   $ 2,800,000        
Restricted Cash, Noncurrent, Statement of Financial Position [Extensible Enumeration]   Deposits and Other Assets Noncurrent   Deposits and Other Assets Noncurrent        
Allowance for credit loss   $ 0 $ 0 $ 0 $ 0      
Allowance for doubtful accounts receivable   15,600,000   15,600,000       $ 15,200,000
Bad debt expense   100,000 $ 5,900,000 800,000 $ 7,800,000      
Income taxes                
Federal income tax statutory rate 21.00%              
Deposits and other assets                
Summary of significant accounting policies                
Bank guarantee denominated in foreign currency   700,000   700,000        
Hopewell Campus | Deposits and other assets                
Summary of significant accounting policies                
Letters of credit   3,000,000   3,000,000        
Warren Building under lease amendment                
Summary of significant accounting policies                
Letters of credit           $ 2,500,000 $ 5,000,000  
Warren Building under lease amendment | Deposits and other assets                
Summary of significant accounting policies                
Letters of credit   $ 10,000,000   $ 10,000,000