Segment Information (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Segment reporting information
Earned insurance and reinsurance premiums	$ 495.8	$ 649.4	$ 982.0	$ 1,315.2
Net investment income	45.8	52.7	95.3	108.5
Net realized and unrealized investment (losses) gains	57.3	11.1	76.6	93.1
Other revenue	(27.8)	(23.6)	(34.8)	(13.5)
Total revenues	571.1	689.6	1,119.1	1,503.3
Loss and loss adjustment expenses	292.4	402.1	665.0	954.0
Insurance and reinsurance acquisition expenses	100.4	134.6	193.1	273.6
Other underwriting expenses	74.8	81.9	151.0	177.5
General and administrative expenses	56.2	52.3	84.2	87.4
Interest expense on debt	12.9	14.7	26.0	31.0
Total expenses	536.7	685.6	1,119.3	1,523.5
Pre-tax income (loss) from continuing operations	34.4	4.0	(0.2)	(20.2)
OneBeacon
Segment reporting information
Earned insurance and reinsurance premiums	265.0	428.3	528.5	881.5
Net investment income	18.7	25.0	39.7	53.3
Net realized and unrealized investment (losses) gains	11.0	(14.4)	34.1	28.0
Other revenue	(11.5)	(10.0)	(10.7)	(10.8)
Total revenues	283.2	428.9	591.6	952.0
Loss and loss adjustment expenses	151.8	261.9	296.4	595.6
Insurance and reinsurance acquisition expenses	55.4	91.3	106.4	188.8
Other underwriting expenses	47.5	58.5	99.9	132.7
General and administrative expenses	2.6	3.0	4.9	7.3
Interest expense on debt	6.0	7.9	12.3	17.0
Total expenses	263.3	422.6	519.9	941.4
Pre-tax income (loss) from continuing operations	19.9	6.3	71.7	10.6
White Mountains Re Segment
Segment reporting information
Earned insurance and reinsurance premiums	230.8	221.1	453.5	433.7
Net investment income	22.7	24.2	46.2	48.3
Net realized and unrealized investment (losses) gains	45.5	25.8	37.9	65.5
Other revenue	(24.0)	(13.3)	(27.6)	(6.9)
Total revenues	275.0	257.8	510.0	540.6
Insurance and reinsurance acquisition expenses	45.0	43.3	86.7	84.8
Other underwriting expenses	27.3	23.4	51.1	44.8
General and administrative expenses	9.2	9.0	15.9	18.4
Interest expense on debt	6.6	6.5	13.1	13.4
Total expenses	228.7	222.4	535.4	519.8
Pre-tax income (loss) from continuing operations	46.3	35.4	(25.4)	20.8
Other Operations
Segment reporting information
Net investment income	4.4	3.5	9.4	6.9
Net realized and unrealized investment (losses) gains	0.8	(0.3)	4.6	(0.4)
Other revenue	7.7	(0.3)	3.5	4.2
Total revenues	12.9	2.9	17.5	10.7
General and administrative expenses	44.4	40.3	63.4	61.7
Interest expense on debt	0.3	0.3	0.6	0.6
Total expenses	44.7	40.6	64.0	62.3
Pre-tax income (loss) from continuing operations	$ (31.8)	$ (37.7)	$ (46.5)	$ (51.6)