Loss and Loss Adjustment Expense Reserves (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($)	Mar. 31, 2023 USD ($)
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (11.4)	$ 621.2
Personal Lines | Personal auto business
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (100.0)	428.0
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	$ 70.0	144.0
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years		42.0
Short-Duration Insurance Contract, Accident Year 2023
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	(50.0)
Short-Duration Insurance Contract, Accident Year 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (15.0)	498.0
Short-Duration Insurance Contract, Accident Year 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years		$ 30.0