Loss and Loss Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Loss and Loss Adjustment, Expense Reserves [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0	$ 7.3
Net (favorable) and unfavorable loss reserve development	(0.1)	(1.9)
Star & Shield Insurance Exchange
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	$ 0.1	0.1
OneBeacon [Member]
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development		$ 1.8
OneBeacon [Member]
Loss and Loss Adjustment, Expense Reserves [Line Items]
Percentage of Quota Share Reinsurance Agreement	50.00%