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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Losses and Loss Expenses Payable [Abstract]
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability	$ 1,135.2	$ 1,234.5	$ 1,135.2	$ 1,234.5	$ 1,146.8	$ 1,255.6
Reinsurance Recoverables, Including Reinsurance Premium Paid	8.4	5.1	8.4	5.1	5.5	3.1
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,126.8	1,229.4	1,126.8	1,229.4	$ 1,141.3	$ 1,252.5
Current Year Claims and Claims Adjustment Expense			466.8	468.4
Prior Year Claims and Claims Adjustment Expense			(37.6)	(38.3)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	$ 234.9	$ 217.8	429.2	430.1
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year			198.6	176.4
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years			245.1	276.8
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid			$ 443.7	$ 453.2