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Reserves for Losses and Adjustment Expenses - Reconciliation of Reinsurance Recoverables on Unpaid Losses (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
May 30, 2022
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]        
Reinsurance recoverable for unpaid claims under retroactive reinsurance, beginning balance $ 3,298.1      
Reinsurance recoverable for unpaid claims under retroactive reinsurance, beginning balance, Ending Balance 4,897.7 $ 3,298.1    
Provision for losses and LAE for claims incurred 28.1 45.1 $ (0.9)  
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 838.1      
Reinsurance recoverable for unpaid claims under retroactive reinsurance, beginning balance, Ending Balance $ 2,132.0 $ 838.1