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Allowance for expected credit losses - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2025
Dec. 31, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Allowance for expected credit losses $ 28.8 $ 28.8
Assets within scope of credit losses | Credit concentration risk | AM Best or S&P    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Concentration risk, percentage 60.00%  
Assets within scope of credit losses | Credit concentration risk | AM Best or S&P | AM Best or S&P, rated A- or better    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Concentration risk, percentage 82.00%