Reserves for Unpaid Losses and Loss Adjustment Expenses (Details 2) (USD $)	12 Months Ended			127 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011
OneBeacon
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Net losses paid by NICO during the period				$ 1,400,000,000
Gross
Beginning balance	1,023,000,000	1,336,300,000	1,568,700,000
Incurred losses and LAE	488,600,000	6,200,000	5,900,000
Paid losses and LAE	(157,500,000)	(319,500,000)	(238,300,000)
Ending balance	1,354,100,000	1,023,000,000	1,336,300,000	1,354,100,000
Pre-NICO Net
Beginning balance	741,100,000	907,400,000	1,002,700,000
Incurred losses and LAE	94,400,000	6,000,000	5,000,000
Paid losses and LAE	(2,700,000)	(172,300,000)	(100,300,000)
Ending balance	832,800,000	741,100,000	907,400,000	832,800,000
Net
Beginning balance	15,600,000	14,100,000	12,000,000
Incurred losses and LAE	6,000,000	6,000,000	5,000,000
Paid losses and LAE	(10,400,000)	(4,500,000)	(2,900,000)
Ending balance	11,200,000	15,600,000	14,100,000	11,200,000
OneBeacon | NICO Cover
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Period during which asbestos and environmental losses have been recovered under the historical third-party reinsurance (in years)	11
Asbestos and environmental losses recovered under the historical third-party reinsurance (as a percent)	47.00%			47.00%
Utilized asbestos and environmental coverage				2,300,000,000
Percentage of utilized coverage from NICO that relates to uncollectible Third-Party Recoverables				8.00%
Net
Beginning balance	2,200,000,000
Increase in A&E reserves ceded to NICO	122,000,000
Ending balance	2,300,000,000			2,300,000,000
OneBeacon | Asbestos
Gross
Beginning balance	904,000,000	985,600,000	1,098,400,000
Incurred losses and LAE	256,800,000		1,000,000
Paid losses and LAE	(86,500,000)	(81,600,000)	(113,800,000)
Ending balance	1,074,300,000	904,000,000	985,600,000	1,074,300,000
Pre-NICO Net
Beginning balance	647,300,000	688,800,000	741,500,000
Incurred losses and LAE	32,200,000
Paid losses and LAE	1,700,000	(41,500,000)	(52,700,000)
Ending balance	681,200,000	647,300,000	688,800,000	681,200,000
Net
Beginning balance	6,400,000	6,500,000	6,500,000
Incurred losses and LAE	(4,000,000)
Paid losses and LAE	(200,000)	(100,000)
Ending balance	2,200,000	6,400,000	6,500,000	2,200,000
OneBeacon | Environmental
Gross
Beginning balance	119,000,000	350,700,000	470,300,000
Incurred losses and LAE	231,800,000	6,200,000	4,900,000
Paid losses and LAE	(71,000,000)	(237,900,000)	(124,500,000)
Ending balance	279,800,000	119,000,000	350,700,000	279,800,000
Pre-NICO Net
Beginning balance	93,800,000	218,600,000	261,200,000
Incurred losses and LAE	62,200,000	6,000,000	5,000,000
Paid losses and LAE	(4,400,000)	(130,800,000)	(47,600,000)
Ending balance	151,600,000	93,800,000	218,600,000	151,600,000
Net
Beginning balance	9,200,000	7,600,000	5,500,000
Incurred losses and LAE	10,000,000	6,000,000	5,000,000
Paid losses and LAE	(10,200,000)	(4,400,000)	(2,900,000)
Ending balance	9,000,000	9,200,000	7,600,000	9,000,000
Sirius
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
A&E three year survival ratio (in years)	11.1	12.5
Gross
Beginning balance	214,300,000	209,200,000	198,200,000
Losses and LAE acquired Central National		13,900,000
Incurred losses and LAE	16,500,000	6,500,000	23,900,000
Paid losses and LAE	(23,600,000)	(15,300,000)	(12,900,000)
Ending balance	207,200,000	214,300,000	209,200,000	207,200,000
Net
Beginning balance	169,600,000	164,500,000	158,600,000
Losses and LAE acquired Central National		12,100,000
Incurred losses and LAE	12,300,000	6,000,000	17,700,000
Paid losses and LAE	(19,200,000)	(13,000,000)	(11,800,000)
Ending balance	162,800,000	169,600,000	164,500,000	162,800,000
Sirius | Asbestos
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Increase (decrease) in reserves	10,300,000
Gross
Beginning balance	191,900,000	187,000,000	173,700,000
Losses and LAE acquired Central National		10,400,000
Incurred losses and LAE	13,600,000	9,000,000	24,700,000
Paid losses and LAE	(20,400,000)	(14,500,000)	(11,400,000)
Ending balance	185,100,000	191,900,000	187,000,000	185,100,000
Net
Beginning balance	151,500,000	146,600,000	138,500,000
Losses and LAE acquired Central National		9,700,000
Incurred losses and LAE	10,300,000	7,300,000	18,400,000
Paid losses and LAE	(15,600,000)	(12,100,000)	(10,300,000)
Ending balance	146,200,000	151,500,000	146,600,000	146,200,000
Sirius | Environmental
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Increase (decrease) in reserves	2,000,000
Gross
Beginning balance	22,400,000	22,200,000	24,500,000
Losses and LAE acquired Central National		3,500,000
Incurred losses and LAE	2,900,000	(2,500,000)	(800,000)
Paid losses and LAE	(3,200,000)	(800,000)	(1,500,000)
Ending balance	22,100,000	22,400,000	22,200,000	22,100,000
Net
Beginning balance	18,100,000	17,900,000	20,100,000
Losses and LAE acquired Central National		2,400,000
Incurred losses and LAE	2,000,000	(1,300,000)	(700,000)
Paid losses and LAE	(3,600,000)	(900,000)	(1,500,000)
Ending balance	$ 16,500,000	$ 18,100,000	$ 17,900,000	$ 16,500,000