Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment reporting information
Earned insurance and reinsurance premiums	$ 512.2	$ 478.3	$ 1,011.2	$ 946.1
Net investment income	40.4	45.8	82.2	95.3
Net realized and unrealized investment gains	(8.1)	42.8	50.5	33.6
Other revenue	(0.5)	(13.3)	30.7	8.2
Total revenues	544.0	553.6	1,174.6	1,083.2
Loss and loss adjustment expenses	265.0	278.5	527.7	636.9
Insurance and reinsurance acquisition expenses	108.7	98.5	217.1	189.2
Other underwriting expenses	79.0	71.4	153.8	144.5
General and administrative expenses	42.1	56.2	87.6	84.2
Interest expense on debt	10.9	12.9	21.8	26.0
Total expenses	505.7	517.5	1,008.0	1,080.8
Pre-tax income (loss) from continuing operations	38.3	36.1	166.6	2.4
OneBeacon
Segment reporting information
Earned insurance and reinsurance premiums	282.0	247.5	554.8	492.6
Net investment income	14.0	18.7	28.7	39.7
Net realized and unrealized investment gains	(11.9)	11.0	17.9	34.1
Other revenue	0.2	(11.5)	0.3	(10.7)
Total revenues	284.3	265.7	601.7	555.7
Loss and loss adjustment expenses	152.5	137.9	301.8	268.3
Insurance and reinsurance acquisition expenses	60.1	53.5	117.5	102.5
Other underwriting expenses	52.0	44.1	100.8	93.4
General and administrative expenses	2.6	2.6	5.3	4.9
Interest expense on debt	4.0	6.0	8.1	12.3
Total expenses	271.2	244.1	533.5	481.4
Pre-tax income (loss) from continuing operations	13.1	21.6	68.2	74.3
Sirius
Segment reporting information
Earned insurance and reinsurance premiums	230.2	230.8	456.4	453.5
Net investment income	17.1	22.7	34.2	46.2
Net realized and unrealized investment gains	18.1	31.0	31.8	(5.1)
Other revenue	(17.6)	(9.5)	0.7	15.4
Total revenues	247.8	275.0	523.1	510.0
Loss and loss adjustment expenses	112.5	140.6	225.9	368.6
Insurance and reinsurance acquisition expenses	48.6	45.0	99.6	86.7
Other underwriting expenses	27.0	27.3	53.0	51.1
General and administrative expenses	6.2	9.2	25.1	15.9
Interest expense on debt	6.6	6.6	13.1	13.1
Total expenses	200.9	228.7	416.7	535.4
Pre-tax income (loss) from continuing operations	46.9	46.3	106.4	(25.4)
Other Operations
Segment reporting information
Earned insurance and reinsurance premiums	0	0	0	0
Net investment income	9.3	4.4	19.3	9.4
Net realized and unrealized investment gains	(14.3)	0.8	0.8	4.6
Other revenue	16.9	7.7	29.7	3.5
Total revenues	11.9	12.9	49.8	17.5
Loss and loss adjustment expenses	0	0	0	0
Insurance and reinsurance acquisition expenses	0	0	0	0
Other underwriting expenses	0	0	0	0
General and administrative expenses	33.3	44.4	57.2	63.4
Interest expense on debt	0.3	0.3	0.6	0.6
Total expenses	33.6	44.7	57.8	64.0
Pre-tax income (loss) from continuing operations	$ (21.7)	$ (31.8)	$ (8.0)	$ (46.5)