Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment reporting information
Percentage of par value of policy reinsured	15.00%								15.00%
Earned insurance premiums									$ 1,188.2	$ 1,185.0	$ 1,120.9
Net investment income									60.8	59.5	59.8
Net investment income (loss) - surplus note interest									0.0	0.0	0.0
Net realized and unrealized investment gains									225.4	78.5	133.9
Other revenue									334.2	131.1	47.0
Total revenues	$ 678.5	$ 334.7	$ 403.6	$ 391.8	$ 405.6	$ 323.2	$ 393.5	$ 331.8	1,808.6	1,454.1	1,361.6
Loss and loss adjustment expenses									708.9	824.0	622.1
Insurance and reinsurance acquisition expenses									220.1	206.2	210.4
Other underwriting expenses									218.6	179.6	205.2
General and administrative expenses									487.5	257.7	149.1
Amortization of Intangible Assets									28.5	11.7	1.4
Interest expense on debt									18.6	15.6	16.2
Total expenses	420.4	429.3	412.7	391.3	471.3	358.7	359.9	293.2	1,653.7	1,483.1	1,203.0
Pre-tax loss	$ 258.1	$ (94.6)	$ (9.1)	$ 0.5	$ (65.7)	$ (35.5)	$ 33.6	$ 38.6	154.9	(29.0)	158.6
Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									458.9	246.0	147.7
OneBeacon
Segment reporting information
Earned insurance premiums									1,176.2	1,177.1	1,120.4
Net investment income									45.9	43.4	43.0
Net investment income (loss) - surplus note interest									0.0	0.0	0.0
Net realized and unrealized investment gains									(35.1)	40.4	49.4
Other revenue									(0.6)	5.8	31.2
Total revenues									1,186.4	1,266.7	1,244.0
Loss and loss adjustment expenses									700.7	815.1	622.1
Insurance and reinsurance acquisition expenses									213.8	203.3	208.9
Other underwriting expenses									218.2	179.2	204.8
Interest expense on debt									13.0	13.0	13.0
Total expenses									1,161.1	1,224.4	1,060.8
Pre-tax loss									25.3	42.3	183.2
OneBeacon | Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									14.1	12.4	10.6
BAM
Segment reporting information
Earned insurance premiums									0.8	0.4	0.1
Net investment income									4.2	5.7	4.7
Net investment income (loss) - surplus note interest									(15.8)	(15.7)	(40.2)
Net realized and unrealized investment gains									0.9	6.6	(9.3)
Other revenue									0.7	0.6	0.4
Total revenues									(9.2)	(2.4)	(44.3)
Loss and loss adjustment expenses									0.0	0.0	0.0
Insurance and reinsurance acquisition expenses									2.3	1.8	1.4
Other underwriting expenses									0.4	0.4	0.4
Interest expense on debt									0.0	0.0	0.0
Total expenses									38.1	38.1	34.3
Pre-tax loss									(47.3)	(40.5)	(78.6)
BAM | Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									$ 35.4	35.9	32.5
HG Global
Segment reporting information
Percentage of par value of policy reinsured	15.00%								15.00%
Earned insurance premiums									$ 2.5	1.4	0.4
Net investment income									1.9	1.4	1.0
Net investment income (loss) - surplus note interest									15.8	15.7	40.2
Net realized and unrealized investment gains									(0.3)	1.7	(2.0)
Other revenue									0.0	0.0	0.0
Total revenues									19.9	20.2	39.6
Loss and loss adjustment expenses									0.0	0.0	0.0
Insurance and reinsurance acquisition expenses									0.6	0.3	0.1
Other underwriting expenses									0.0	0.0	0.0
Interest expense on debt									0.0	0.0	0.0
Total expenses									2.0	1.9	1.5
Pre-tax loss									17.9	18.3	38.1
HG Global | Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									1.4	1.6	1.4
Other Segments
Segment reporting information
Earned insurance premiums									8.7	6.1	0.0
Net investment income									8.8	9.0	11.1
Net investment income (loss) - surplus note interest									0.0	0.0	0.0
Net realized and unrealized investment gains									259.9	29.8	95.8
Other revenue									334.1	124.7	15.4
Total revenues									611.5	169.6	122.3
Loss and loss adjustment expenses									8.2	8.9	0.0
Insurance and reinsurance acquisition expenses									3.4	0.8	0.0
Other underwriting expenses									0.0	0.0	0.0
Interest expense on debt									5.6	2.6	3.2
Total expenses									452.5	218.7	106.4
Pre-tax loss									159.0	(49.1)	15.9
Other Segments | Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses									408.0	196.1	103.2
General and Administrative Expense [Member]
Segment reporting information
Amortization of Intangible Assets									28.6	11.7	1.4
General and Administrative Expense [Member] | OneBeacon
Segment reporting information
Amortization of Intangible Assets									1.3	1.4	1.4
General and Administrative Expense [Member] | BAM
Segment reporting information
Amortization of Intangible Assets									0.0	0.0	0.0
General and Administrative Expense [Member] | HG Global
Segment reporting information
Amortization of Intangible Assets									0.0	0.0	0.0
General and Administrative Expense [Member] | Other Segments
Segment reporting information
Amortization of Intangible Assets									27.3	10.3	$ 0.0
Symetra | Other Segments
Segment reporting information
Net realized and unrealized investment gains									258.8
Tranzact [Member] | Other Segments
Segment reporting information
Other revenue									186.9	43.2
General and administrative expenses									167.3	37.4
MediaAlpha [Member] | Other Segments
Segment reporting information
Other revenue									105.5	65.3
General and administrative expenses									$ 99.0	$ 60.6