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Summary of significant accounting policies - Narrative (Details)
$ in Thousands
 3 Months Ended 9 Months Ended
Dec. 22, 2017
Sep. 30, 2025
USD ($)
segment
Sep. 30, 2024
USD ($)
segment
Sep. 30, 2025
USD ($)
segment
Sep. 30, 2024
USD ($)
segment
Dec. 31, 2028
USD ($)
Jul. 01, 2027
USD ($)
Dec. 31, 2024
USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                
Number of reporting segments | segment       1 1      
Number of operating segments | segment   1 1 1 1      
Allowance for credit loss   $ 0 $ 0 $ 0 $ 0      
Increase in allowance for credit losses   0 0 0 0      
Allowance for doubtful accounts receivable   11,600   11,600       $ 2,300
Bad debt expense   800 $ 700 8,600 $ 500      
Income taxes                
Federal income tax statutory rate 21.00%              
Deposits and other assets                
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                
Bank guarantee denominated in foreign currency   700   700        
Hopewell Campus | Deposits and other assets                
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                
Letters of credit   3,000   3,000        
Warren Building under lease amendment                
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                
Letters of credit           $ 2,500 $ 5,000  
Warren Building under lease amendment | Deposits and other assets                
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                
Letters of credit   $ 10,000   $ 10,000