Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment reporting information
Earned insurance premiums	$ 284.9	$ 307.2	$ 896.0	$ 879.1
Net investment income	16.8	13.8	42.6	44.7
Net investment income (loss) - surplus note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	(43.9)	(27.5)	(33.9)	64.4
Other revenue	76.9	29.7	225.4	60.3
Total revenues	334.7	323.2	1,130.1	1,048.5
Losses and LAE	170.0	185.5	534.1	527.3
Insurance and Reinsurance Acquisition Expenses	55.3	55.2	166.1	153.0
Other Underwriting Expense	56.5	51.3	165.5	152.6
General and administrative expenses	142.6	63.0	354.1	168.3
Interest expense	4.9	3.7	13.5	10.6
Total expenses	429.3	358.7	1,233.3	1,011.8
Pre-tax (loss) income	(94.6)	(35.5)	(103.2)	36.7
OneBeacon
Segment reporting information
Earned insurance premiums	281.4	305.4	887.3	872.8
Net investment income	12.4	10.8	33.0	33.4
Net investment income (loss) - surplus note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	(29.9)	(16.0)	(29.7)	24.8
Other revenue	3.9	1.3	(0.4)	3.3
Total revenues	267.8	301.5	890.2	934.3
Losses and LAE	167.5	185.2	527.9	520.7
Insurance and Reinsurance Acquisition Expenses	53.8	54.6	161.2	150.9
Other Underwriting Expense	56.4	51.2	165.2	152.3
General and administrative expenses	3.7	2.4	12.0	9.3
Interest expense	3.2	3.2	9.7	9.7
Total expenses	284.6	296.6	876.0	842.9
Pre-tax (loss) income	(16.8)	4.9	14.2	91.4
HG Global
Segment reporting information
Earned insurance premiums	0.6	0.4	1.7	0.9
Net investment income	0.5	0.4	1.4	1.0
Net investment income (loss) - surplus note interest	4.0		11.9
Net realized and unrealized investment gains (losses)	0.6	(0.5)	0.3	1.1
Other revenue	0.0	0.0	0.0	0.0
Total revenues	5.7	4.2	15.3	14.8
Losses and LAE	0.0	0.0	0.0	0.0
Insurance and Reinsurance Acquisition Expenses	0.1	0.0	0.3	0.2
Other Underwriting Expense	0.0	0.0	0.0	0.0
General and administrative expenses	0.4	0.4	1.2	1.2
Interest expense	0.0	0.0	0.0	0.0
Total expenses	0.5	0.4	1.5	1.4
Pre-tax (loss) income	5.2		13.8
BAM
Segment reporting information
Earned insurance premiums	0.3	0.1	0.6	0.3
Net investment income	1.1	1.4	2.9	4.2
Net investment income (loss) - surplus note interest	(4.0)	(3.9)	(11.9)	(11.8)
Net realized and unrealized investment gains (losses)	1.7	(1.4)	2.8	5.4
Other revenue	0.2	0.2	0.5	0.5
Total revenues	(0.7)	(3.6)	(5.1)	(1.4)
Losses and LAE	0.0	0.0	0.0	0.0
Insurance and Reinsurance Acquisition Expenses	0.4	0.5	1.8	1.4
Other Underwriting Expense	0.1	0.1	0.3	0.3
General and administrative expenses	9.4	8.9	26.3	26.6
Interest expense	0.0	0.0	0.0	0.0
Total expenses	9.9	9.5	28.4	28.3
Pre-tax (loss) income	(10.6)	(13.1)	(33.5)	(29.7)
Other Operations
Segment reporting information
Earned insurance premiums	2.6	1.3	6.4	5.1
Net investment income	2.8	1.2	5.3	6.1
Net investment income (loss) - surplus note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	(16.3)	(9.6)	(7.3)	33.1
Other revenue	72.8	28.2	225.3	56.5
Total revenues	61.9	21.1	229.7	100.8
Losses and LAE	2.5	0.3	6.2	6.6
Insurance and Reinsurance Acquisition Expenses	1.0	0.1	2.8	0.5
Other Underwriting Expense	0.0	0.0	0.0	0.0
General and administrative expenses	129.1	51.3	314.6	131.2
Interest expense	1.7	0.5	3.8	0.9
Total expenses	134.3	52.2	327.4	139.2
Pre-tax (loss) income	$ (72.4)	$ (31.1)	$ (97.7)	$ (38.4)