Property and Casualty Insurance Reserves - Reserve Activity (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Incurred Losses and LAE related to:
Incurred Losses and LAE Related to Prior Years	$ 27.4	$ 158.5
Property, Liability and Casualty Insurance Product Line
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross of Reinsurance at Beginning of Year	2,680.5	2,756.9
Less: Reinsurance Recoverables at Beginning of Year	27.8	39.6
Property and Casualty Insurance Reserves, Net of Reinsurance at Beginning of Year	2,652.7	2,717.3
Incurred Losses and LAE related to:
Incurred Losses and LAE Related to Current Year	2,053.0	2,660.5
Incurred Losses and LAE Related to Prior Years	27.4	158.5
Total Incurred Losses and LAE	2,080.4	2,819.0
Paid Losses and LAE related to:
Paid Losses and LAE Related to Current Year	945.0	1,411.8
Paid Losses and LAE Related to Prior Years	1,227.6	1,428.9
Total Paid Losses and LAE	2,172.6	2,840.7
Property and Casualty Insurance Reserves, Net of Reinsurance at End of Period	2,560.5	2,695.6
Plus: Reinsurance Recoverables at End of Period	26.4	28.9
Property and Casualty Insurance Reserves, Gross of Reinsurance at End of Period	$ 2,586.9	$ 2,724.5