Reserves for Unpaid Losses and Loss Adjustment Expenses (LAE Reserve Activity) (Details) (Sirius Group, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Loss and Loss Adjustment, Expense Reserves [Line Items]
Liability for asbestos claims, net, period increase (decrease)	$ 8.0	$ 11.8	$ 33.0	$ 46.4
Liabilty for environmental claims, net, period increase (decrease)	1.6	0.8	0.5
Gross
Beginning balance	227.8	228.6	207.2
Losses and LAE acquired	25.0	14.0	11.7
Incurred losses and LAE	9.0	12.9	46.7
Paid losses and LAE	(23.3)	(27.7)	(37.0)
Ending balance	238.5	227.8	228.6	207.2
Net
Beginning balance	193.9	189.4	162.7
Losses and LAE acquired Central National	25.0	14.0	11.7
Incurred losses and LAE	9.6	12.6	45.9
Liability for unpaid claims and claims adjustment expense	(18.3)	(22.1)	(30.9)
Ending balance	210.2	193.9	189.4	162.7
Asbestos
Gross
Beginning balance	207.4	208.2	185.1
Losses and LAE acquired	22.9	13.0	11.0
Incurred losses and LAE	7.4	12.1	46.8
Paid losses and LAE	(21.9)	(25.9)	(34.7)
Ending balance	215.8	207.4	208.2
Net
Beginning balance	178.7	174.2	146.2
Losses and LAE acquired Central National	22.9	13.0	11.0
Incurred losses and LAE	8.0	11.8	46.4
Liability for unpaid claims and claims adjustment expense	(16.8)	(20.3)	(29.4)
Ending balance	192.8	178.7	174.2
Environmental
Gross
Beginning balance	20.4	20.4	22.1
Losses and LAE acquired	2.1	1.0	0.7
Incurred losses and LAE	1.6	0.8	(0.1)
Paid losses and LAE	(1.4)	(1.8)	(2.3)
Ending balance	22.7	20.4	20.4
Net
Beginning balance	15.2	15.2	16.5
Losses and LAE acquired Central National	2.1	1.0	0.7
Incurred losses and LAE	1.6	0.8	(0.5)
Liability for unpaid claims and claims adjustment expense	(1.5)	(1.8)	(1.5)
Ending balance	17.4	15.2	15.2
WM Solutions
Loss and Loss Adjustment, Expense Reserves [Line Items]
Liability for asbestos claims, net, period increase (decrease)	22.9	13.0	11.0
Liabilty for environmental claims, net, period increase (decrease)	$ 2.1	$ 1.0	$ 0.7