Loss and Loss Adjustment Expense Reserves (Details) $ in Millions	6 Months Ended
	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (62.5)	$ 1,110.5
Personal Lines | Personal auto business
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (235.0)	870.0
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	$ 140.0	224.0
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	30.0
Short-Duration Insurance Contract, Accident Year 2023
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	(60.0)
Short-Duration Insurance Contract, Accident Year 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (20.0)	910.0
Short-Duration Insurance Contract, Accident Year 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years		$ 81.0