Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment reporting information
Earned insurance and reinsurance premiums	$ 537.3	$ 503.4	$ 1,031.6	$ 997.0
Net investment income	23.8	29.6	47.1	54.3
Net investment income (loss) - surplus note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	(61.1)	113.9	42.3	177.7
Other revenue	87.5	6.4	123.3	3.0
Total revenues	587.5	653.3	1,244.3	1,232.0
Losses and LAE	288.3	282.7	553.7	512.0
Insurance and Reinsurance Acquisition Expenses	105.3	98.9	200.9	194.0
Other Underwriting Expense	80.7	81.9	162.2	163.3
General and administrative expenses	108.1	70.3	224.5	120.6
Interest expense	10.9	10.0	22.0	20.1
Total expenses	593.3	543.8	1,163.3	1,010.0
Pre-tax income (loss)	(5.8)	109.5	81.0	222.0
OneBeacon
Segment reporting information
Earned insurance and reinsurance premiums	319.3	290.9	605.9	567.4
Net investment income	10.1	12.2	20.6	22.6
Net investment income (loss) - surplus note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	(14.0)	21.9	0.2	40.8
Other revenue	(1.2)	1.0	(4.3)	2.0
Total revenues	314.2	326.0	622.4	632.8
Losses and LAE	194.5	186.1	360.4	335.5
Insurance and Reinsurance Acquisition Expenses	56.4	49.6	107.4	96.3
Other Underwriting Expense	52.9	51.7	108.8	101.1
General and administrative expenses	4.2	3.6	8.3	6.9
Interest expense	3.3	3.3	6.5	6.5
Total expenses	311.3	294.3	591.4	546.3
Pre-tax income (loss)	2.9	31.7	31.0	86.5
Sirius Group
Segment reporting information
Earned insurance and reinsurance premiums	215.5	209.6	420.5	425.1
Net investment income	9.3	11.8	17.3	20.5
Net investment income (loss) - surplus note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	(42.7)	58.4	37.2	86.5
Other revenue	5.4	(19.2)	(22.0)	(24.5)
Total revenues	187.5	260.6	453.0	507.6
Losses and LAE	92.1	91.4	189.6	170.2
Insurance and Reinsurance Acquisition Expenses	47.5	48.8	90.1	96.2
Other Underwriting Expense	27.7	30.2	53.2	62.0
General and administrative expenses	6.8	6.9	13.0	15.3
Interest expense	6.6	6.6	13.4	13.2
Total expenses	180.7	183.9	359.3	356.9
Pre-tax income (loss)	6.8	76.7	93.7	150.7
HG Global
Segment reporting information
Earned insurance and reinsurance premiums	0.6	0.3	1.1	0.5
Net investment income	0.5	0.3	0.9	0.6
Net investment income (loss) - surplus note interest	3.9	4.0	7.9	7.9
Net realized and unrealized investment gains (losses)	(1.4)	0.9	(0.3)	1.6
Other revenue	0.0	0.0	0.0	0.0
Total revenues	3.6	5.5	9.6	10.6
Losses and LAE	0.0	0.0	0.0	0.0
Insurance and Reinsurance Acquisition Expenses	0.1	0.1	0.2	0.2
Other Underwriting Expense	0.0	0.0	0.0	0.0
General and administrative expenses	0.4	0.4	0.8	0.8
Interest expense	0.0	0.0	0.0	0.0
Total expenses	0.5	0.5	1.0	1.0
Pre-tax income (loss)	3.1	5.0	8.6	9.6
BAM
Segment reporting information
Earned insurance and reinsurance premiums	0.1	0.1	0.3	0.2
Net investment income	0.8	1.4	1.8	2.8
Net investment income (loss) - surplus note interest	(3.9)	(4.0)	(7.9)	(7.9)
Net realized and unrealized investment gains (losses)	(1.9)	3.8	1.1	6.8
Other revenue	0.2	0.1	0.3	0.3
Total revenues	(4.7)	1.4	(4.4)	2.2
Losses and LAE	0.0	0.0	0.0	0.0
Insurance and Reinsurance Acquisition Expenses	0.6	0.5	1.4	0.9
Other Underwriting Expense	0.1	0.1	0.2	0.2
General and administrative expenses	8.7	8.8	16.9	17.7
Interest expense	0.0	0.0	0.0	0.0
Total expenses	9.4	9.4	18.5	18.8
Pre-tax income (loss)	(14.1)	(8.0)	(22.9)	(16.6)
Other Operations
Segment reporting information
Earned insurance and reinsurance premiums	1.8	2.5	3.8	3.8
Net investment income	3.1	3.9	6.5	7.8
Net investment income (loss) - surplus note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	(1.1)	28.9	4.1	42.0
Other revenue	83.1	24.5	149.3	25.2
Total revenues	86.9	59.8	163.7	78.8
Losses and LAE	1.7	5.2	3.7	6.3
Insurance and Reinsurance Acquisition Expenses	0.7	(0.1)	1.8	0.4
Other Underwriting Expense	0.0	(0.1)	0.0	0.0
General and administrative expenses	88.0	50.6	185.5	79.9
Interest expense	1.0	0.1	2.1	0.4
Total expenses	91.4	55.7	193.1	87.0
Pre-tax income (loss)	$ (4.5)	$ 4.1	$ (29.4)	$ (8.2)