Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment reporting information
Earned insurance and reinsurance premiums									$ 2,063.6	$ 1,924.5	$ 2,029.0
Net investment income									153.6	184.5	208.9
Net investment income (loss) - surplus note interest									0
Net realized and unrealized investment gains									118.2	74.1	77.6
Other revenue									100.3	(10.0)	18.2
Revenues (including realized gains and losses)	566.4	697.4	542.3	629.7	593.4	501.7	552.4	525.6	2,435.7	2,173.1	2,333.7
Loss and loss adjustment expenses									1,193.9	1,174.3	1,216.6
Insurance and reinsurance acquisition expenses									430.2	402.2	419.6
Other underwriting expenses									321.8	268.1	295.9
General and administrative expenses									182.2	175.3	155.0
Interest expense on debt									44.8	55.2	57.3
Total expenses	617.2	562.3	504.7	488.8	495.0	500.5	516.6	563.0	2,172.9	2,075.1	2,144.4
Pre-tax income	(50.8)	135.1	37.6	140.9	98.4	1.2	35.8	(37.4)	262.8	98.0	189.3
Percentage of par value of policy reinsured	15.00%								15.00%
Sirius Group Segment [Member]
Segment reporting information
Earned insurance and reinsurance premiums									931.6	912.3	847.9
Loss and loss adjustment expenses									543.9	626.0	531.0
OneBeacon
Segment reporting information
Earned insurance and reinsurance premiums									1,132.0	1,012.2	1,181.1
Net investment income									53.6	71.4	96.6
Net investment income (loss) - surplus note interest									0
Net realized and unrealized investment gains									55.7	10.6	74.6
Other revenue									(0.5)	(12.4)	(0.6)
Revenues (including realized gains and losses)									1,240.8	1,081.8	1,351.7
Loss and loss adjustment expenses									650.0	548.3	685.6
Insurance and reinsurance acquisition expenses									249.4	221.2	252.1
Other underwriting expenses									205.2	162.3	196.1
General and administrative expenses									13.4	9.8	12.9
Interest expense on debt									16.9	20.5	29.6
Total expenses									1,134.9	962.1	1,176.3
Pre-tax income									105.9	119.7	175.4
Sirius Group
Segment reporting information
Earned insurance and reinsurance premiums									931.6	912.3	847.9
Net investment income									65.0	89.9	96.5
Net investment income (loss) - surplus note interest									0
Net realized and unrealized investment gains									17.3	53.2	(14.8)
Other revenue									70.6	4.1	40.9
Revenues (including realized gains and losses)									1,084.5	1,059.5	970.5
Loss and loss adjustment expenses									543.9	626.0	531.0
Insurance and reinsurance acquisition expenses									180.8	181.0	167.5
Other underwriting expenses									116.4	105.8	99.8
General and administrative expenses									45.9	34.1	31.6
Interest expense on debt									26.2	31.6	26.6
Total expenses									913.2	978.5	856.5
Pre-tax income									171.3	81.0	114.0
BAM
Segment reporting information
Earned insurance and reinsurance premiums									0
Net investment income									1.9
Net investment income (loss) - surplus note interest									(18.4)
Net realized and unrealized investment gains									0
Other revenue									0
Revenues (including realized gains and losses)									(16.5)
Loss and loss adjustment expenses									0
Insurance and reinsurance acquisition expenses									0
Other underwriting expenses									0.2
General and administrative expenses									19.6
Interest expense on debt									0
Total expenses									19.8
HG Global
Segment reporting information
Earned insurance and reinsurance premiums									0
Net investment income									0.3
Net investment income (loss) - surplus note interest									18.4
Net realized and unrealized investment gains									0
Other revenue									0
Revenues (including realized gains and losses)									18.7
Loss and loss adjustment expenses									0
Insurance and reinsurance acquisition expenses									0
Other underwriting expenses									0
General and administrative expenses									4.5
Interest expense on debt									0
Total expenses									4.5
Pre-tax income									14.2
Percentage of par value of policy reinsured	15.00%								15.00%
Other Operations
Segment reporting information
Earned insurance and reinsurance premiums									0	0	0
Net investment income									32.8	23.2	15.8
Net investment income (loss) - surplus note interest									0
Net realized and unrealized investment gains									45.2	10.3	17.8
Other revenue									30.2	(1.7)	(22.1)
Revenues (including realized gains and losses)									108.2	31.8	11.5
Loss and loss adjustment expenses									0	0	0
Insurance and reinsurance acquisition expenses									0	0	0
Other underwriting expenses									0	0	0
General and administrative expenses									98.8	131.4	110.5
Interest expense on debt									1.7	3.1	1.1
Total expenses									100.5	134.5	111.6
Pre-tax income									7.7	(102.7)	(100.1)
Non-controlling Interests | Build America Mutual | BAM
Segment reporting information
Pre-tax income									$ (36.3)