Reserves for Unpaid Losses and Loss Adjustment Expenses (LAE Reserve Activity) (Details) (Sirius Group, USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Gross
Beginning balance	$ 207.2	$ 207.2	$ 214.3	$ 209.2
Losses and LAE acquired		11.7	0	13.9
Incurred losses and LAE		46.7	16.5	6.5
Paid losses and LAE		(37.0)	(23.6)	(15.3)
Ending balance		228.6	207.2	214.3
Net
Beginning balance	162.7	162.7	169.6	164.5
Losses and LAE acquired Central National		11.7	0	12.1
Incurred losses and LAE		45.9	12.3	6.0
Liability for unpaid claims and claims adjustment expense	(8.5)	(30.9)	(19.2)	(13.0)
Ending balance		189.4	162.7	169.6
Asbestos Issue
Gross
Beginning balance	185.1	185.1	191.9	187.0
Losses and LAE acquired		11.0	0	10.4
Incurred losses and LAE		46.8	13.6	9.0
Paid losses and LAE		(34.7)	(20.4)	(14.5)
Ending balance		208.2	185.1	191.9
Net
Beginning balance	146.2	146.2	151.5	146.6
Losses and LAE acquired Central National		11.0	0	9.7
Incurred losses and LAE		46.4	10.3	7.3
Liability for unpaid claims and claims adjustment expense		(29.4)	(15.6)	(12.1)
Ending balance		174.2	146.2	151.5
Environmental Issue
Gross
Beginning balance	22.1	22.1	22.4	22.2
Losses and LAE acquired		0.7	0	3.5
Incurred losses and LAE		(0.1)	2.9	(2.5)
Paid losses and LAE		(2.3)	(3.2)	(0.8)
Ending balance		20.4	22.1	22.4
Net
Beginning balance	16.5	16.5	18.1	17.9
Losses and LAE acquired Central National		0.7	0	2.4
Incurred losses and LAE		(0.5)	2.0	(1.3)
Liability for unpaid claims and claims adjustment expense		(1.5)	(3.6)	(0.9)
Ending balance		$ 15.2	$ 16.5	$ 18.1