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, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Losses and Loss Expenses Payable [Abstract]
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability	$ 1,217.3	$ 1,083.5	$ 1,181.6	$ 1,053.0
Reinsurance Recoverables, Including Reinsurance Premium Paid	4.8	6.5	3.6	5.9
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,212.5	1,077.0	$ 1,178.0	$ 1,047.1
Current Year Claims and Claims Adjustment Expense	243.7	215.4
Prior Year Claims and Claims Adjustment Expense	(9.7)	9.6
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	234.0	225.0
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	60.1	52.3
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	139.4	142.8
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	$ 199.5	$ 195.1