Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment reporting information
Net investment income	$ 24.7	$ 27.3	$ 78.1	$ 84.5
Earned insurance and reinsurance premiums	538.6	500.4	1,535.6	1,493.3
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	20.8	28.2	198.5	66.1
Other revenue	5.3	18.2	8.3	46.9
Total revenues	589.4	574.1	1,820.5	1,690.8
Losses and LAE	289.1	278.3	801.1	797.2
Insurance and reinsurance acquisition expenses	105.3	106.7	299.3	281.0
Other underwriting expenses	81.0	80.4	244.3	244.0
General and administrative expenses	69.3	41.5	189.9	125.5
Interest expense	10.3	11.9	30.4	32.4
Total expenses	555.0	518.8	1,565.0	1,480.1
Pre-tax income (loss)	34.4	55.3	255.5	210.7
OneBeacon
Segment reporting information
Net investment income	10.4	10.1	32.1	30.9
Earned insurance and reinsurance premiums	305.4	278.9	872.8	846.2
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	(16.0)	17.0	24.8	19.9
Other revenue	1.3	5.5	3.3	30.1
Total revenues	301.1	311.5	933.0	927.1
Losses and LAE	185.2	167.8	520.7	473.7
Insurance and reinsurance acquisition expenses	54.6	53.6	150.9	160.9
Other underwriting expenses	51.2	46.8	152.3	150.7
General and administrative expenses	2.4	4.1	9.3	11.0
Interest expense	3.2	3.3	9.7	9.8
Total expenses	296.6	275.6	842.9	806.1
Pre-tax income (loss)	4.5	35.9	90.1	121.0
Sirius Group
Segment reporting information
Net investment income	9.8	13.5	30.3	38.0
Earned insurance and reinsurance premiums	231.4	221.4	656.5	646.9
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	49.1	(24.7)	135.6	(4.9)
Other revenue	(22.8)	16.8	(47.3)	11.3
Total revenues	267.5	227.0	775.1	691.3
Losses and LAE	103.6	110.5	273.8	323.5
Insurance and reinsurance acquisition expenses	50.1	52.5	146.3	119.0
Other underwriting expenses	29.7	33.5	91.7	93.0
General and administrative expenses	6.3	7.5	21.6	23.0
Interest expense	6.6	6.6	19.8	19.7
Total expenses	196.3	210.6	553.2	578.2
Pre-tax income (loss)	71.2	16.4	221.9	113.1
HG Global
Segment reporting information
Net investment income	0.4	0.2	1.0	0.7
Earned insurance and reinsurance premiums	0.4	0.1	0.9	0.2
Net investment income (loss) - surplus note interest	3.9	10.1	11.8	30.2
Net realized and unrealized investment gains (losses)	(0.5)	0.5	1.1	(1.7)
Other revenue	0	0	0	0
Total revenues	4.2	10.9	14.8	29.4
Losses and LAE	0	0	0	0
Insurance and reinsurance acquisition expenses	0	0.1	0.2	0.1
Other underwriting expenses	0	0	0	0
General and administrative expenses	0.4	0.3	1.2	1.1
Interest expense	0	0	0	0
Total expenses	0.4	0.4	1.4	1.2
Pre-tax income (loss)	3.8	10.5	13.4	28.2
BAM
Segment reporting information
Net investment income	1.4	1.1	4.2	3.3
Earned insurance and reinsurance premiums	0.1	0	0.3	0
Net investment income (loss) - surplus note interest	(3.9)	(10.1)	(11.8)	(30.2)
Net realized and unrealized investment gains (losses)	(1.4)	2.4	5.4	(8.1)
Other revenue	0.2	0.2	0.5	0.3
Total revenues	(3.6)	(6.4)	(1.4)	(34.7)
Losses and LAE	0	0	0	0
Insurance and reinsurance acquisition expenses	0.5	0.5	1.4	1.0
Other underwriting expenses	0.1	0.1	0.3	0.3
General and administrative expenses	8.9	8.1	26.6	24.2
Interest expense	0	0	0	0
Total expenses	9.5	8.7	28.3	25.5
Pre-tax income (loss)	(13.1)	(15.1)	(29.7)	(60.2)
Other Operations
Segment reporting information
Net investment income	2.7	2.4	10.5	11.6
Earned insurance and reinsurance premiums	1.3	0	5.1	0
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	(10.4)	33.0	31.6	60.9
Other revenue	26.6	(4.3)	51.8	5.2
Total revenues	20.2	31.1	99.0	77.7
Losses and LAE	0.3	0	6.6	0
Insurance and reinsurance acquisition expenses	0.1	0	0.5	0
Other underwriting expenses	0	0	0	0
General and administrative expenses	51.3	21.5	131.2	66.2
Interest expense	0.5	2.0	0.9	2.9
Total expenses	52.2	23.5	139.2	69.1
Pre-tax income (loss)	$ (32.0)	$ 7.6	$ (40.2)	$ 8.6