Loss and Loss Adjustment Expense Reserves Details (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	$ (12.1)	$ (11.6)	$ (19.0)	$ (25.6)
Accretion of fair value adjustment to net loss and LAE reserves	0.1	0.2	0.5	1.5
Adjustment to loss and loss adjustment expense reserves, before tax	4.2		4.2
Sirius Group
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	(19.2)	(15.4)	(35.3)	(25.5)
Star & Shield Insurance Exchange
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	(0.2)		2.0
OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	$ 7.3	$ 3.8	$ 14.3	$ (0.1)