Property and Casualty Insurance Reserves - Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Incurred Losses and LAE related to:
Incurred Losses and LAE Related to Prior Years	$ 8.8	$ 41.9
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	686.1	944.0
Incurred Losses and LAE Related to Prior Years	8.8	41.9
Total Incurred Losses and LAE	694.9	985.9
Paid Losses and LAE related to:
Paid Losses and LAE Related to Current Year	213.9	325.5
Paid Losses and LAE Related to Prior Years	562.9	690.9
Total Paid Losses and LAE	776.8	1,016.4
Property and Casualty Insurance Reserves, Net of Reinsurance at End of Period	2,570.8	2,686.8
Plus Reinsurance Recoverables at End of Period	26.4	34.3
Property and Casualty Insurance Reserves, Gross of Reinsurance at End of Period	$ 2,597.2	$ 2,721.1