Property and Casualty Insurance Reserves - Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross of Reinsurance at Beginning of Year	$ 1,969.8
Incurred Losses and LAE Related to:
Incurred Losses and LAE Related to Prior Years	0.9	$ (21.4)
Paid Losses and LAE Related to:
Property and Casualty Insurance Reserves - Gross of Reinsurance at End of Period	1,941.6
Property, Liability and Casualty Insurance Product Line
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross of Reinsurance at Beginning of Year	1,969.8	1,874.9
Less Reinsurance Recoverables at Beginning of Year	65.6	101.9
Property and Casualty Insurance Reserves - Net of Reinsurance at Beginning of Year	1,904.2	1,773.0
Incurred Losses and LAE Related to:
Incurred Losses and LAE Related to Current Year	738.1	687.6
Incurred Losses and LAE Related to Prior Years	0.9	(21.4)
Total Incurred Losses and LAE	739.0	666.2
Paid Losses and LAE Related to:
Paid Losses and LAE Related to Current Year	257.0	223.5
Paid Losses and LAE Related to Prior Years	505.8	445.4
Total Paid Losses and LAE	762.8	668.9
Property and Casualty Insurance Reserves - Net of Reinsurance at End of Period	1,880.4	1,770.3
Plus Reinsurance Recoverables at End of Period	61.2	98.4
Property and Casualty Insurance Reserves - Gross of Reinsurance at End of Period	$ 1,941.6	$ 1,868.7