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Loss and Loss Adjustment Expense Reserves (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Liability for Claims and Claims Adjustment Expense [Line Items]    
Prior years $ 1,147.0 $ 52.8
Personal Lines | Personal auto business    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Prior years 866.0 (28.0)
Personal Lines | Personal auto business | Direct    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Prior years   (25.0)
Commercial Lines    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Prior years 277.0 77.0
Short-Duration Insurance Contract, Accident Year 2022    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Prior years 929.0  
Short-Duration Insurance Contract, Accident Year 2021    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Prior years $ 108.0 45.0
Short Duration Insurance Contract Accident Years 2019 and Prior    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Prior years   29.0
Short-Duration Insurance Contract, Accident Year 2020    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Prior years   $ (21.0)