Loss and Loss Adjustment Expense Reserves Details (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended	6 Months Ended		3 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 One Beacon	Jun. 30, 2012 One Beacon	Jun. 30, 2013 One Beacon	Jun. 30, 2012 One Beacon	Jun. 30, 2013 Sirius Group [Member]	Jun. 30, 2013 Sirius Group [Member]	Jun. 30, 2012 Sirius Group [Member]	Mar. 31, 2012 Scandinavian Reinsurance Company [Member]
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net favorable and unfavorable loss reserve development	$ (15.4)	$ (3.5)	$ (14.0)	$ (11.1)	$ (1.0)	$ (3.4)	$ (3.9)	$ (5.3)	$ (14.4)	$ (10.1)	$ (5.8)
Accretion of fair value adjustment to net loss and LAE reserves	0.1	1.2	1.3	8.3
Accretion of fair value adjustment to net loss and LAE reserves increase												5.0
Adjustment to loss and loss adjustment expense reserves, before tax	$ 4.4		$ 4.4