Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment reporting information
Earned insurance premiums	$ 497.5	$ 510.5	$ 992.9	$ 1,008.5
Net investment income	28.7	40.4	57.2	82.2
Net investment income - surplus note income	0		0
Net realized and unrealized investment gains (losses)	(37.3)	(8.1)	37.9	50.5
Other revenue	0.4	(0.5)	28.7	30.7
Total revenues	489.3	542.3	1,116.7	1,171.9
Losses and loss adjustment expenses	274.6	263.9	518.9	513.7
Insurance and reinsurance acquisition expenses	76.1	109.5	174.3	218.6
Other underwriting expenses	84.5	78.3	163.6	151.8
General and administrative expenses	40.2	42.1	84.0	87.6
Interest expense on debt	10.3	10.9	20.5	21.8
Total expenses	485.7	504.7	961.3	993.5
Pre-tax income (loss)	3.6	37.6	155.4	178.4
OneBeacon
Segment reporting information
Earned insurance premiums	280.8	280.3	567.3	552.1
Net investment income	11.4	14.0	20.8	28.7
Net investment income - surplus note income	0		0
Net realized and unrealized investment gains (losses)	(25.5)	(11.9)	2.9	17.9
Other revenue	0.6	0.2	24.6	0.3
Total revenues	267.3	282.6	615.6	599.0
Losses and loss adjustment expenses	157.0	151.4	305.9	287.8
Insurance and reinsurance acquisition expenses	52.5	60.9	107.3	119.0
Other underwriting expenses	54.6	51.3	103.9	98.8
General and administrative expenses	2.9	2.6	6.9	5.3
Interest expense on debt	3.3	4.0	6.5	8.1
Total expenses	270.3	270.2	530.5	519.0
Pre-tax income (loss)	(3.0)	12.4	85.1	80.0
Sirius
Segment reporting information
Earned insurance premiums	216.6	230.2	425.5	456.4
Net investment income	11.4	17.1	24.5	34.2
Net investment income - surplus note income	0		0
Net realized and unrealized investment gains (losses)	6.7	18.1	19.8	31.8
Other revenue	(16.0)	(17.6)	(5.5)	0.7
Total revenues	218.7	247.8	464.3	523.1
Losses and loss adjustment expenses	117.6	112.5	213.0	225.9
Insurance and reinsurance acquisition expenses	23.2	48.6	66.5	99.6
Other underwriting expenses	29.8	27.0	59.5	53.0
General and administrative expenses	6.0	6.2	15.5	25.1
Interest expense on debt	6.5	6.6	13.1	13.1
Total expenses	183.1	200.9	367.6	416.7
Pre-tax income (loss)	35.6	46.9	96.7	106.4
HG Global
Segment reporting information
Earned insurance premiums	0.1		0.1
Net investment income	0.2		0.5
Net investment income - surplus note income	10.0		20.1
Net realized and unrealized investment gains (losses)	(2.0)		(2.2)
Other revenue	0		0
Total revenues	8.3		18.5
Losses and loss adjustment expenses	0		0
Insurance and reinsurance acquisition expenses	0		0
Other underwriting expenses	0		0
General and administrative expenses	0.5		0.8
Interest expense on debt	0		0
Total expenses	0.5		0.8
Pre-tax income (loss)	7.8		17.7
BAM
Segment reporting information
Earned insurance premiums	0		0
Net investment income	1.1		2.2
Net investment income - surplus note income	(10.0)		(20.1)
Net realized and unrealized investment gains (losses)	(9.4)		(10.5)
Other revenue	0		0.1
Total revenues	(18.3)		(28.3)
Losses and loss adjustment expenses	0		0
Insurance and reinsurance acquisition expenses	0.4		0.5
Other underwriting expenses	0.1		0.2
General and administrative expenses	7.9		16.1
Interest expense on debt	0		0
Total expenses	8.4		16.8
Pre-tax income (loss)	(26.7)		(45.1)
Other Operations
Segment reporting information
Earned insurance premiums	0	0	0	0
Net investment income	4.6	9.3	9.2	19.3
Net investment income - surplus note income	0		0
Net realized and unrealized investment gains (losses)	(7.1)	(14.3)	27.9	0.8
Other revenue	15.8	16.9	9.5	29.7
Total revenues	13.3	11.9	46.6	49.8
Losses 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	22.9	33.3	44.7	57.2
Interest expense on debt	0.5	0.3	0.9	0.6
Total expenses	23.4	33.6	45.6	57.8
Pre-tax income (loss)	$ (10.1)	$ (21.7)	$ 1.0	$ (8.0)