Segment Information (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment reporting information
Earned insurance and reinsurance premiums	$ 490.6	$ 498.1	$ 1,436.7	$ 1,772.9
Net investment income	42.8	48.9	138.1	157.4
Net realized and unrealized investment gains (losses)	1.7	11.6	78.3	104.7
Other revenue	(33.8)	(7.6)	(68.6)	(21.1)
Total revenues	501.3	551.0	1,584.5	2,013.9
Loss and loss adjustment expenses	277.1	253.4	914.0	1,175.9
Insurance and reinsurance acquisition expenses	107.0	102.1	296.2	370.9
Other underwriting expenses	67.0	75.0	211.5	245.9
General and administrative expenses	36.2	32.1	120.4	119.5
Interest expense on debt	12.8	13.2	38.8	44.2
Total expenses	500.1	475.8	1,580.9	1,956.4
Pre-tax income (loss) from continuing operations	1.2	75.2	3.6	57.5
OneBeacon
Segment reporting information
Earned insurance and reinsurance premiums	258.6	293.6	751.2	1,134.7
Net investment income	16.1	21.6	55.8	74.9
Net realized and unrealized investment gains (losses)	(47.4)	51.6	(13.3)	79.6
Other revenue	0.1	12.7	(10.6)	1.9
Total revenues	227.4	379.5	783.1	1,291.1
Loss and loss adjustment expenses	146.6	155.2	414.9	719.3
Insurance and reinsurance acquisition expenses	58.6	64.3	161.1	248.3
Other underwriting expenses	38.7	50.5	132.1	176.6
General and administrative expenses	2.5	2.1	7.4	9.4
Interest expense on debt	4.1	6.4	16.4	23.4
Total expenses	250.5	278.5	731.9	1,177.0
Pre-tax income (loss) from continuing operations	(23.1)	101.0	51.2	114.1
Sirius Group.
Segment reporting information
Earned insurance and reinsurance premiums	232.0	204.5	685.5	638.2
Net investment income	22.1	23.0	68.3	71.3
Net realized and unrealized investment gains (losses)	64.7	(51.8)	102.6	13.7
Other revenue	(28.1)	43.3	(55.7)	36.4
Total revenues	290.7	219.0	800.7	759.6
Loss and loss adjustment expenses	130.5	98.2	499.1	456.6
Insurance and reinsurance acquisition expenses	48.4	37.8	135.1	122.6
Other underwriting expenses	28.3	24.5	79.4	69.3
General and administrative expenses	7.4	4.8	23.3	23.2
Interest expense on debt	6.5	6.7	19.6	20.1
Total expenses	221.1	172.0	756.5	691.8
Pre-tax income (loss) from continuing operations	69.6	47.0	44.2	67.8
Other Operations
Segment reporting information
Net investment income	4.6	4.3	14.0	11.2
Net realized and unrealized investment gains (losses)	(15.6)	11.8	(11.0)	11.4
Other revenue	(5.8)	(63.6)	(2.3)	(59.4)
Total revenues	(16.8)	(47.5)	0.7	(36.8)
General and administrative expenses	26.3	25.2	89.7	86.9
Interest expense on debt	2.2	0.1	2.8	0.7
Total expenses	28.5	25.3	92.5	87.6
Pre-tax income (loss) from continuing operations	$ (45.3)	$ (72.8)	$ (91.8)	$ (124.4)