Property and Casualty Insurance Reserves - Reserve Activity (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross of Reinsurance at Beginning of Year	$ 1,982.5
Incurred Losses and LAE Related to:
Incurred Losses and LAE Related to Prior Years	107.9	$ 28.3
Paid Losses and LAE Related to:
Property and Casualty Insurance Reserves - Gross of Reinsurance at End of Period	2,596.2
Property, Liability and Casualty Insurance Product Line
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross of Reinsurance at Beginning of Year	1,982.5	1,969.8
Less Reinsurance Recoverables at Beginning of Year	50.1	65.6
Property and Casualty Insurance Reserves - Net of Reinsurance at Beginning of Year	1,932.4	1,904.2
Property and Casualty Insurance Reserves Acquired, Net of Reinsurance	211.1	0.0
Incurred Losses and LAE Related to:
Incurred Losses and LAE Related to Current Year	2,885.7	2,132.3
Incurred Losses and LAE Related to Prior Years	107.9	28.3
Total Incurred Losses and LAE	2,993.6	2,160.6
Paid Losses and LAE Related to:
Paid Losses and LAE Related to Current Year	1,503.9	1,133.7
Paid Losses and LAE Related to Prior Years	1,080.4	1,032.9
Total Paid Losses and LAE	2,584.3	2,166.6
Property and Casualty Insurance Reserves - Net of Reinsurance at End of Period	2,552.8	1,898.2
Plus Reinsurance Recoverables at End of Period	43.4	51.8
Property and Casualty Insurance Reserves - Gross of Reinsurance at End of Period	$ 2,596.2	$ 1,950.0