Loss and Loss Adjustment Expense Reserves (Details) $ in Millions	9 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (199.4)	$ 1,147.0
Personal Lines | Personal auto business
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (375.0)	866.0
Personal Lines | Personal auto business | Agency
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of prior year claims and claims adjustment expense	0.60
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 100.0	277.0
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	75.0
Short-Duration Insurance Contract, Accident Year 2023
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	(160.0)
Short-Duration Insurance Contract, Accident Year 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (5.0)	929.0
Short-Duration Insurance Contract, Accident Year 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years		$ 108.0