Property and Casualty Insurance Reserves - Reserve Activity (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross of Reinsurance at Beginning of Year	$ 2,756.9
Incurred Losses and Loss Adjustment Expenses (“ LAE”) Related to:
Incurred Losses and LAE Related to Prior Years	158.5	$ (22.5)
Paid Losses and LAE Related to:
Property and Casualty Insurance Reserves, Gross of Reinsurance at End of Period	2,724.5
Property, Liability and Casualty Insurance Product Line
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross of Reinsurance at Beginning of Year	2,756.9	2,772.7
Less Reinsurance Recoverables at Beginning of Year	39.6	41.9
Property and Casualty Insurance Reserves, Net of Reinsurance at Beginning of Year	2,717.3	2,730.8
Incurred Losses and Loss Adjustment Expenses (“ LAE”) Related to:
Incurred Losses and LAE Related to Current Year	2,660.5	3,125.0
Incurred Losses and LAE Related to Prior Years	158.5	(22.5)
Total Incurred Losses and LAE	2,819.0	3,102.5
Paid Losses and LAE Related to:
Paid Losses and LAE Related to Current Year	1,411.8	1,748.9
Paid Losses and LAE Related to Prior Years	1,428.9	1,406.6
Total Paid Losses and LAE	2,840.7	3,155.5
Property and Casualty Insurance Reserves, Net of Reinsurance at End of Period	2,695.6	2,677.8
Plus Reinsurance Recoverables at End of Period	28.9	42.5
Property and Casualty Insurance Reserves, Gross of Reinsurance at End of Period	$ 2,724.5	$ 2,720.3