Loss and Loss Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Loss and Loss Adjustment, Expense Reserves [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0		$ 6.8	$ 0.0	$ 14.1
Net (favorable) and unfavorable loss reserve development	15.3		(0.3)	15.2	(2.2)
Loss and LAE incurred relating to current year losses	166.7		196.5	327.9	366.3
Star & Shield Insurance Exchange
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	(0.1)		$ (0.3)	(0.2)	(0.4)
OneBeacon [Member]
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	15.4				$ 1.8
Loss and LAE incurred relating to current year losses	$ 20.0
OneBeacon [Member]
Loss and Loss Adjustment, Expense Reserves [Line Items]
Percentage of Quota Share Reinsurance Agreement		50.00%
Net (favorable) and unfavorable loss reserve development				$ 15.4