Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment reporting information
Net investment income	$ 29.1	$ 28.7	$ 53.4	$ 57.2
Earned insurance and reinsurance premiums	503.4	497.5	997.0	992.9
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	113.9	(37.3)	177.7	37.9
Other revenue	6.4	0.4	3.0	28.7
Total revenues	652.8	489.3	1,231.1	1,116.7
Losses and LAE	282.7	274.6	512.0	518.9
Insurance and reinsurance acquisition expenses	98.9	76.1	194.0	174.3
Other underwriting expenses	81.9	84.5	163.3	163.6
General and administrative expenses	70.3	40.2	120.6	84.0
Interest expense	10.0	10.3	20.1	20.5
Total expenses	543.8	485.7	1,010.0	961.3
Pre-tax income (loss)	109.0	3.6	221.1	155.4
OneBeacon
Segment reporting information
Net investment income	11.7	11.4	21.7	20.8
Earned insurance and reinsurance premiums	290.9	280.8	567.4	567.3
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	21.9	(25.5)	40.8	2.9
Other revenue	1.0	0.6	2.0	24.6
Total revenues	325.5	267.3	631.9	615.6
Losses and LAE	186.1	157.0	335.5	305.9
Insurance and reinsurance acquisition expenses	49.6	52.5	96.3	107.3
Other underwriting expenses	51.7	54.6	101.1	103.9
General and administrative expenses	3.6	2.9	6.9	6.9
Interest expense	3.3	3.3	6.5	6.5
Total expenses	294.3	270.3	546.3	530.5
Pre-tax income (loss)	31.2	(3.0)	85.6	85.1
Sirius Group
Segment reporting information
Net investment income	11.8	11.4	20.5	24.5
Earned insurance and reinsurance premiums	209.6	216.6	425.1	425.5
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	58.4	6.7	86.5	19.8
Other revenue	(19.2)	(16.0)	(24.5)	(5.5)
Total revenues	260.6	218.7	507.6	464.3
Losses and LAE	91.4	117.6	170.2	213.0
Insurance and reinsurance acquisition expenses	48.8	23.2	96.2	66.5
Other underwriting expenses	30.2	29.8	62.0	59.5
General and administrative expenses	6.9	6.0	15.3	15.5
Interest expense	6.6	6.5	13.2	13.1
Total expenses	183.9	183.1	356.9	367.6
Pre-tax income (loss)	76.7	35.6	150.7	96.7
HG Global
Segment reporting information
Net investment income	0.3	0.2	0.6	0.5
Earned insurance and reinsurance premiums	0.3	0.1	0.5	0.1
Net investment income (loss) - surplus note interest	4.0	10.0	7.9	20.1
Net realized and unrealized investment gains (losses)	0.9	(2.0)	1.6	(2.2)
Other revenue	0	0	0	0
Total revenues	5.5	8.3	10.6	18.5
Losses and LAE	0	0	0	0
Insurance and reinsurance acquisition expenses	0.1	0	0.2	0
Other underwriting expenses	0	0	0	0
General and administrative expenses	0.4	0.5	0.8	0.8
Interest expense	0	0	0	0
Total expenses	0.5	0.5	1.0	0.8
Pre-tax income (loss)	5.0	7.8	9.6	17.7
BAM
Segment reporting information
Net investment income	1.4	1.1	2.8	2.2
Earned insurance and reinsurance premiums	0.1	0	0.2	0
Net investment income (loss) - surplus note interest	(4.0)	(10.0)	(7.9)	(20.1)
Net realized and unrealized investment gains (losses)	3.8	(9.4)	6.8	(10.5)
Other revenue	0.1	0	0.3	0.1
Total revenues	1.4	(18.3)	2.2	(28.3)
Losses and LAE	0	0	0	0
Insurance and reinsurance acquisition expenses	0.5	0.4	0.9	0.5
Other underwriting expenses	0.1	0.1	0.2	0.2
General and administrative expenses	8.8	7.9	17.7	16.1
Interest expense	0	0	0	0
Total expenses	9.4	8.4	18.8	16.8
Pre-tax income (loss)	(8.0)	(26.7)	(16.6)	(45.1)
Other Operations
Segment reporting information
Net investment income	3.9	4.6	7.8	9.2
Earned insurance and reinsurance premiums	2.5	0	3.8	0
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	28.9	(7.1)	42.0	27.9
Other revenue	24.5	15.8	25.2	9.5
Total revenues	59.8	13.3	78.8	46.6
Losses and LAE	5.2	0	6.3	0
Insurance and reinsurance acquisition expenses	(0.1)	0	0.4	0
Other underwriting expenses	(0.1)	0	0	0
General and administrative expenses	50.6	22.9	79.9	44.7
Interest expense	0.1	0.5	0.4	0.9
Total expenses	55.7	23.4	87.0	45.6
Pre-tax income (loss)	$ 4.1	$ (10.1)	$ (8.2)	$ 1.0