Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment reporting information
Earned insurance and reinsurance premiums	$ 536.8	$ 491.1	$ 1,545.3	$ 1,433.5
Net investment income	37.6	42.8	119.8	138.1
Net realized and unrealized investment gains (losses)	72.7	2.9	123.2	36.4
Other revenue	50.3	(35.0)	81.0	(28.3)
Revenues	697.4	501.8	1,869.3	1,579.7
Loss and loss adjustment expenses	308.1	280.2	821.7	920.4
Insurance and reinsurance acquisition expenses	107.6	107.0	326.2	296.6
Other underwriting expenses	76.6	64.3	228.4	203.9
General and administrative expenses	58.7	36.2	146.3	120.4
Interest expense on debt	11.3	12.8	33.1	38.8
Total expenses	562.3	500.5	1,555.7	1,580.1
Pre-tax income (loss)	135.1	1.3	313.6	(0.4)
OneBeacon
Segment reporting information
Earned insurance and reinsurance premiums	293.9	259.1	846.0	748.0
Net investment income	12.8	16.1	41.5	55.8
Net realized and unrealized investment gains (losses)	40.0	(47.4)	57.9	(13.3)
Other revenue	(0.4)	0.1	(0.1)	(12.2)
Revenues	346.3	227.9	945.3	778.3
Loss and loss adjustment expenses	164.7	149.7	452.5	421.3
Insurance and reinsurance acquisition expenses	66.6	58.6	185.6	161.5
Other underwriting expenses	47.4	36.0	146.2	124.5
General and administrative expenses	4.4	2.5	9.6	7.4
Interest expense on debt	4.1	4.1	12.2	16.4
Total expenses	287.2	250.9	806.1	731.1
Pre-tax income (loss)	59.1	(23.0)	139.2	47.2
Sirius
Segment reporting information
Earned insurance and reinsurance premiums	242.9	232.0	699.3	685.5
Net investment income	16.7	22.1	50.9	68.3
Net realized and unrealized investment gains (losses)	(8.9)	65.9	22.9	60.7
Other revenue	48.2	(29.3)	48.9	(13.8)
Revenues	298.9	290.7	822.0	800.7
Loss and loss adjustment expenses	143.4	130.5	369.2	499.1
Insurance and reinsurance acquisition expenses	41.0	48.4	140.6	135.1
Other underwriting expenses	29.1	28.3	82.1	79.4
General and administrative expenses	10.2	7.4	35.4	23.3
Interest expense on debt	6.5	6.5	19.6	19.6
Total expenses	230.2	221.1	646.9	756.5
Pre-tax income (loss)	68.7	69.6	175.1	44.2
Other Operations
Segment reporting information
Earned insurance and reinsurance premiums	0	0	0	0
Net investment income	8.1	4.6	27.4	14.0
Net realized and unrealized investment gains (losses)	41.6	(15.6)	42.4	(11.0)
Other revenue	2.5	(5.8)	32.2	(2.3)
Revenues	52.2	(16.8)	102.0	0.7
Loss and loss adjustment expenses	0	0	0	0
Insurance and reinsurance acquisition expenses	0	0	0	0
Other underwriting expenses	0.1	0	0.1	0
General and administrative expenses	44.1	26.3	101.3	89.7
Interest expense on debt	0.7	2.2	1.3	2.8
Total expenses	44.9	28.5	102.7	92.5
Pre-tax income (loss)	$ 7.3	$ (45.3)	$ (0.7)	$ (91.8)