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Reserves for Losses and Adjustment Expenses - Reconciliation of Reinsurance Recoverables on Unpaid Losses (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
May 20, 2022
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]            
Reinsurance recoverable for unpaid claims under retroactive reinsurance, beginning balance $ 4,577.8 $ 4,897.7 $ 4,897.7      
Reinsurance recoverable for unpaid claims under retroactive reinsurance, beginning balance, Ending Balance     4,577.8 $ 4,897.7    
Provision for losses and LAE for claims incurred 39.7 10.5 60.8 28.1 $ 45.1  
Reinsurance Contracts [Axis]: LPT            
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]            
Reinsurance recoverables, premium paid and recognized           $ 1,840.1
Reinsurance Contracts [Axis]: Retroactive Reinsurance            
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]            
Reinsurance recoverable for unpaid claims under retroactive reinsurance, beginning balance $ 1,627.4 $ 2,132.0 2,132.0      
Reinsurance recoverable for unpaid claims under retroactive reinsurance, beginning balance, Ending Balance     $ 1,627.4 $ 2,132.0