Losses and Loss Expenses Payable Losses and Loss Expenses Payable - Schedule of Activity in Liability for Unpaid Losses and Loss Expenses Payable (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Abstract]
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability	$ 1,091.3	$ 1,092.7	$ 1,091.3	$ 1,092.7	$ 1,066.5	$ 1,146.8
Reinsurance Recoverables, Including Reinsurance Premium Paid	35.2	7.2	35.2	7.2	13.6	5.5
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,056.1	1,085.5	1,056.1	1,085.5	$ 1,052.9	$ 1,141.3
Current Year Claims and Claims Adjustment Expense			779.8	692.7
Prior Year Claims and Claims Adjustment Expense			(17.6)	(55.1)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	$ 251.6	$ 208.4	762.2	637.6
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year			420.8	366.4
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years			338.2	327.0
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid			$ 759.0	$ 693.4