Loss and Loss Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended	9 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Jun. 30, 2016	Sep. 30, 2016	Sep. 30, 2015
Loss and Loss Adjustment, Expense Reserves [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0			$ 3.9		$ 0.0	$ 18.0
Net (favorable) and unfavorable loss reserve development	0.1			(0.4)		15.3	(2.6)
Loss and LAE incurred relating to current year losses	164.9			170.4		492.8	536.7
Star & Shield Insurance Exchange
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	$ 0.1			(0.4)		$ (0.1)	(0.8)
OneBeacon [Member]
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development		$ 20.0		$ 15.4			$ 1.8
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