Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment reporting information
Earned insurance premiums	$ 495.4	$ 498.0
Net investment income	28.5	41.8
Net investment income - surplus note income	0
Net realized and unrealized investment gains (losses)	75.2	58.6
Other revenue	28.3	31.2
Total revenues	627.4	629.6
Losses and loss adjustment expenses	244.3	249.8
Insurance and reinsurance acquisition expenses	98.2	109.1
Other underwriting expenses	79.1	73.5
General and administrative expenses	43.8	45.5
Interest expense on debt	10.2	10.9
Total expenses	475.6	488.8
Pre-tax income (loss)	151.8	140.8
OneBeacon
Segment reporting information
Earned insurance premiums	286.5	271.8
Net investment income	9.4	14.7
Net investment income - surplus note income	0
Net realized and unrealized investment gains (losses)	28.4	29.8
Other revenue	24.0	0.1
Total revenues	348.3	316.4
Losses and loss adjustment expenses	148.9	136.4
Insurance and reinsurance acquisition expenses	54.8	58.1
Other underwriting expenses	49.3	47.5
General and administrative expenses	4.0	2.7
Interest expense on debt	3.2	4.1
Total expenses	260.2	248.8
Pre-tax income (loss)	88.1	67.6
Sirius
Segment reporting information
Earned insurance premiums	208.9	226.2
Net investment income	13.1	17.1
Net investment income - surplus note income	0
Net realized and unrealized investment gains (losses)	13.1	13.7
Other revenue	10.5	18.3
Total revenues	245.6	275.3
Losses and loss adjustment expenses	95.4	113.4
Insurance and reinsurance acquisition expenses	43.3	51.0
Other underwriting expenses	29.7	26.0
General and administrative expenses	9.5	18.9
Interest expense on debt	6.6	6.5
Total expenses	184.5	215.8
Pre-tax income (loss)	61.1	59.5
HG Global
Segment reporting information
Earned insurance premiums	0
Net investment income	0.3
Net investment income - surplus note income	10.1
Net realized and unrealized investment gains (losses)	(0.2)
Other revenue	0
Total revenues	10.2
Losses and loss adjustment expenses	0
Insurance and reinsurance acquisition expenses	0
Other underwriting expenses	0
General and administrative expenses	0.3
Interest expense on debt	0
Total expenses	0.3
Pre-tax income (loss)	9.9
BAM
Segment reporting information
Earned insurance premiums	0
Net investment income	1.1
Net investment income - surplus note income	(10.1)
Net realized and unrealized investment gains (losses)	(1.1)
Other revenue	0.1
Total revenues	(10.0)
Losses and loss adjustment expenses	0
Insurance and reinsurance acquisition expenses	0.1
Other underwriting expenses	0.1
General and administrative expenses	8.2
Interest expense on debt	0
Total expenses	8.4
Pre-tax income (loss)	(18.4)
Other Operations
Segment reporting information
Earned insurance premiums	0	0
Net investment income	4.6	10.0
Net investment income - surplus note income	0
Net realized and unrealized investment gains (losses)	35.0	15.1
Other revenue	(6.3)	12.8
Total revenues	33.3	37.9
Losses and loss adjustment expenses	0	0
Insurance and reinsurance acquisition expenses	0	0
Other underwriting expenses	0	0
General and administrative expenses	21.8	23.9
Interest expense on debt	0.4	0.3
Total expenses	22.2	24.2
Pre-tax income (loss)	$ 11.1	$ 13.7