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
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Losses and Loss Expenses Payable [Abstract]
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability	$ 1,247.6	$ 1,217.6	$ 1,255.6	$ 1,181.6
Reinsurance Recoverables, Including Reinsurance Premium Paid	2.1	4.8	3.1	3.6
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,245.5	1,212.8	$ 1,252.5	$ 1,178.0
Current Year Claims and Claims Adjustment Expense	232.7	244.0
Prior Year Claims and Claims Adjustment Expense	(20.4)	(9.7)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	212.3	234.3
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	63.0	60.1
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	156.3	139.4
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	$ 219.3	$ 199.5