Property and Casualty Insurance Reserves - Reserve Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross of Reinsurance at Beginning of Year	$ 1,969.8
Less Reinsurance Recoverables at Beginning of Year	122.6
Incurred Losses and LAE related to:
Incurred Losses and LAE Related to Prior Years	36.4	$ (71.1)	$ (7.4)
Paid Losses and LAE related to:
Plus Reinsurance Recoverables at End of Year	50.1	122.6
Property and Casualty Insurance Reserves, Gross of Reinsurance at End of Year	1,982.5	1,969.8
Property and Liability Insurance
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross of Reinsurance at Beginning of Year	1,969.8	1,874.9	1,016.8
Less Reinsurance Recoverables at Beginning of Year	65.6	101.9	53.1
Property and Casualty Insurance Reserves, Net of Reinsurance at Beginning of Year	1,904.2	1,773.0	963.7
Property and Casualty Insurance Reserves Acquired, Net of Reinsurance		3.6	695.1
Incurred Losses and LAE related to:
Incurred Losses and LAE Related to Current Year	2,873.6	2,879.5	2,093.4
Incurred Losses and LAE Related to Prior Years	36.4	(71.1)	(7.4)
Total Incurred Losses and LAE	2,910.0	2,808.4	2,086.0
Paid Losses and LAE related to:
Paid Losses and LAE Related to Current Year	1,679.1	1,682.1	1,300.8
Paid Losses and LAE Related to Prior Years	1,202.7	998.7	671.0
Total Paid Losses and LAE	2,881.8	2,680.8	1,971.8
Property and Casualty Insurance Reserves, Net of Reinsurance at End of Year	1,932.4	1,904.2	1,773.0
Plus Reinsurance Recoverables at End of Year	50.1	65.6	101.9
Property and Casualty Insurance Reserves, Gross of Reinsurance at End of Year	$ 1,982.5	$ 1,969.8	$ 1,874.9