Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment reporting information
Percentage of par value of policy reinsured	15.00%								15.00%
Earned insurance and reinsurance premiums									$ 2,058.9	$ 1,987.3	$ 2,063.6
Net investment income									105.0	110.9	153.6
Net investment income (loss) - surplus note interest									0	0	0
Net realized and unrealized investment gains									283.9	161.7	118.2
Other revenue									62.4	57.5	100.3
Total revenues	689.7	589.4	652.8	578.3	626.6	574.1	489.3	627.4	2,510.2	2,317.4	2,435.7
Loss and loss adjustment expenses									1,169.3	1,040.5	1,193.9
Insurance and reinsurance acquisition expenses									399.8	376.9	430.2
Other underwriting expenses									309.3	331.3	321.8
General and administrative expenses									288.2	181.3	182.2
Interest expense on debt									41.9	42.5	44.8
Expenses	643.5	555.0	543.8	466.2	492.4	518.8	485.7	475.6	2,208.5	1,972.5	2,172.9
Pre-tax loss	46.2	34.4	109.0	112.1	134.2	55.3	3.6	151.8	301.7	344.9	262.8
OneBeacon
Segment reporting information
Earned insurance and reinsurance premiums									1,177.1	1,120.4	1,132.0
Net investment income									41.7	41.1	53.6
Net investment income (loss) - surplus note interest									0	0	0
Net realized and unrealized investment gains									40.4	49.4	55.7
Other revenue									5.8	31.2	(0.5)
Total revenues									1,265.0	1,242.1	1,240.8
Loss and loss adjustment expenses									815.1	622.1	650.0
Insurance and reinsurance acquisition expenses									203.3	208.9	249.4
Other underwriting expenses									179.2	204.8	205.2
General and administrative expenses									13.8	12.0	13.4
Interest expense on debt									13.0	13.0	16.9
Expenses									1,224.4	1,060.8	1,134.9
Pre-tax loss									40.6	181.3	105.9
Sirius Group
Segment reporting information
Earned insurance and reinsurance premiums									873.9	866.4	931.6
Net investment income									41.1	48.8	65.0
Net investment income (loss) - surplus note interest									0	0	0
Net realized and unrealized investment gains									209.2	26.7	17.3
Other revenue									(62.4)	16.8	70.6
Total revenues									1,061.8	958.7	1,084.5
Loss and loss adjustment expenses									345.3	418.4	543.9
Insurance and reinsurance acquisition expenses									193.6	166.5	180.8
Other underwriting expenses									129.7	126.1	116.4
General and administrative expenses									30.5	32.2	45.9
Interest expense on debt									26.3	26.3	26.2
Expenses									725.4	769.5	913.2
Pre-tax loss									336.4	189.2	171.3
BAM
Segment reporting information
Earned insurance and reinsurance premiums									0.4	0.1	0
Net investment income									5.7	4.7	1.9
Net investment income (loss) - surplus note interest									(15.7)	(40.2)	(18.4)
Net realized and unrealized investment gains									6.6	(9.3)	0
Other revenue									0.6	0.4	0
Total revenues									(2.4)	(44.3)	(16.5)
Loss and loss adjustment expenses									0	0	0
Insurance and reinsurance acquisition expenses									1.8	1.4	0
Other underwriting expenses									0.4	0.4	0.2
General and administrative expenses									35.9	32.5	19.6
Interest expense on debt									0	0	0
Expenses									38.1	34.3	19.8
Pre-tax loss									(40.5)	(78.6)	(36.3)
Other Segments
Segment reporting information
Earned insurance and reinsurance premiums									6.1	0	0
Net investment income									15.1	15.3	32.8
Net investment income (loss) - surplus note interest									0	0	0
Net realized and unrealized investment gains									26.0	96.9	45.2
Other revenue									118.4	9.1	30.2
Total revenues									165.6	121.3	108.2
Loss and loss adjustment expenses									8.9	0	0
Insurance and reinsurance acquisition expenses									0.8	0	0
Other underwriting expenses									0	0	0
General and administrative expenses									206.4	103.2	98.8
Interest expense on debt									2.6	3.2	1.7
Expenses									218.7	106.4	100.5
Pre-tax loss									(53.1)	14.9	7.7
HG Global
Segment reporting information
Percentage of par value of policy reinsured	15.00%								15.00%
Earned insurance and reinsurance premiums									1.4	0.4	0
Net investment income									1.4	1.0	0.3
Net investment income (loss) - surplus note interest									15.7	40.2	18.4
Net realized and unrealized investment gains									1.7	(2.0)	0
Other revenue									0	0	0
Total revenues									20.2	39.6	18.7
Loss and loss adjustment expenses									0	0	0
Insurance and reinsurance acquisition expenses									0.3	0.1	0
Other underwriting expenses									0	0	0
General and administrative expenses									1.6	1.4	4.5
Interest expense on debt									0	0	0
Expenses									1.9	1.5	4.5
Pre-tax loss									$ 18.3	$ 38.1	$ 14.2