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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Losses and Loss Expenses Payable [Abstract]
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability	$ 1,280.5	$ 1,165.6	$ 1,280.5	$ 1,165.6	$ 1,181.6	$ 1,053.0
Reinsurance Recoverables, Including Reinsurance Premium Paid	4.9	4.1	4.9	4.1	3.6	5.9
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,275.6	1,161.5	1,275.6	1,161.5	$ 1,178.0	$ 1,047.1
Current Year Claims and Claims Adjustment Expense			752.5	686.8
Prior Year Claims and Claims Adjustment Expense			(34.6)	33.4
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	$ 253.5	$ 232.3	717.9	720.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year			302.0	286.6
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years			318.3	319.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid			$ 620.3	$ 605.8