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Related party transactions - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Dec. 31, 2024
Related Party Transaction [Line Items]      
Premiums written, gross $ 984.7 $ 906.6  
Insurance and reinsurance balances receivable, net 2,240.8   $ 2,054.4
Reinsurance balances payable 1,707.5   1,781.6
Related Party      
Related Party Transaction [Line Items]      
Premiums written, gross 6.5 $ 17.0  
Insurance and reinsurance balances receivable, net 152.0   127.2
Reinsurance balances payable $ 0.0   $ 0.0