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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Losses and Loss Expenses Payable [Abstract]
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability	$ 1,198.7	$ 1,280.8	$ 1,198.7	$ 1,280.8	$ 1,255.6	$ 1,181.6
Reinsurance Recoverables, Including Reinsurance Premium Paid	5.0	4.9	5.0	4.9	3.1	3.6
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,193.7	1,275.9	1,193.7	1,275.9	$ 1,252.5	$ 1,178.0
Current Year Claims and Claims Adjustment Expense			678.7	752.8
Prior Year Claims and Claims Adjustment Expense			57.6	(34.6)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	$ 191.0	$ 253.5	621.1	718.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year			314.1	302.0
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years			365.8	318.3
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid			$ 679.9	$ 620.3