Discontinued Operations (Details 3) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
Revenues
Earned premiums	$ 293,900,000	$ 259,100,000	$ 846,000,000	$ 748,000,000
Net other revenues	(400,000)		(100,000)	(12,200,000)
Total revenues	346,300,000	227,800,000	945,300,000	778,300,000
Expenses
Loss and loss adjustment expenses	164,700,000	149,700,000	452,500,000	421,300,000
Policy acquisition expenses	66,600,000	58,600,000	185,600,000	161,500,000
Other underwriting expenses	47,400,000	36,000,000	146,200,000	124,500,000
Total expenses	287,100,000	250,900,000	806,100,000	731,100,000
Income tax benefit	(14,200,000)	11,100,000	(28,100,000)	(1,900,000)
Loss from discontinued operations, net of tax	(15,800,000)	(2,500,000)	(24,800,000)	(1,500,000)
Reinsurance recoverable on paid losses	2,000,000		2,000,000			16,500,000
Reinsurance recoverable on unpaid losses	41,300,000	1,939,300,000	41,300,000	1,939,300,000	2,040,700,000	2,167,500,000	1,825,900,000	1,893,200,000
Reinsurance Recoverables	43,300,000		43,300,000			2,184,000,000
Third party recoverables which reinsurer would pay if they are unable to collect from third party reinsurers	198,300,000		198,300,000
Reinsurance recoverable from various third party reinsurers, guaranteed by the third party reinsurer under reinsurance agreement	28,700,000		28,700,000
The percentage ceded under quota share reinsurance agreement (as a percent)			100.00%
NICO Cover
Expenses
Number of reinsurance contracts the seller caused OneBeacon to purchase immediately prior to the entity's acquisition of OneBeacon	2		2
National Indemnity Company and General Reinsurance Corporation
Expenses
Reinsurance Recoverables	1,449,200,000		1,449,200,000
Percentage of total paid and unpaid reinsurance recoverables (as a percent)	68.00%		68.00%
Hanover Insurance Company
Expenses
Reinsurance Recoverables	62,400,000		62,400,000
Percentage of total paid and unpaid reinsurance recoverables (as a percent)	3.00%		3.00%
Tokio Marine and Nichido Fire
Expenses
Reinsurance Recoverables	54,700,000		54,700,000
Percentage of total paid and unpaid reinsurance recoverables (as a percent)	3.00%		3.00%
Munich Reinsurance America
Expenses
Reinsurance Recoverables	23,200,000		23,200,000
Percentage of total paid and unpaid reinsurance recoverables (as a percent)	1.00%		1.00%
Tower Insurance Company
Expenses
Reinsurance Recoverables	24,700,000		24,700,000
Percentage of total paid and unpaid reinsurance recoverables (as a percent)	1.00%		1.00%
National Indemnity Company [Member] | NICO Cover
Expenses
Maximum cover under reinsurance contract			2,500,000,000
Ceded estimated incurred losses	2,300,000,000		2,300,000,000
Reinsurance receivables paid by the reinsurer			1,500,000,000
General Reinsurance Corporation [Member] | GRC Cover
Expenses
Reinsurance recoverable on unpaid losses	409,300,000		409,300,000
Adverse loss reserve development on losses occurring in years 2000 and prior	400,000,000		400,000,000
Reserves ceded as of the date of the OneBeacon acquisition	170,000,000		170,000,000
Ceded estimated incurred losses	562,000,000		562,000,000
General Reinsurance Corporation [Member] | GRC Cover | Maximum
Expenses
Maximum cover under reinsurance contract			570,000,000
Runoff
Discontinued Operations
Loss on sale of discontinued operation, pre-tax	140,700,000
Loss on sale of discontinued operation, net of tax	91,500,000
Loss and LAE, after tax	9,000,000
Expenses
Reinsurance recoverable on paid losses	17,600,000		17,600,000
Reinsurance recoverable on unpaid losses	1,956,600,000		1,956,600,000
Purchase accounting adjustments	153,400,000		153,400,000
Reinsurance recoverable on unpaid losses in the event when runoff transaction closes	36,700,000		36,700,000
Runoff | OneBeacon
Discontinued Operations
Loss on sale of discontinued operation, net of tax	107,000,000
Loss and LAE, after tax	9,000,000
Expenses
After tax underwriting losses included in discontinued operations	6,500,000
Estimated loss on sale of discontinued operation, net of tax	91,500,000
AutoOne business
Discontinued Operations
Loss on sale of discontinued operation, net of tax		18,200,000
Net assets held for sale		25,000,000		25,000,000
AutoOne business | OneBeacon
Expenses
Estimated loss on sale of discontinued operation, net of tax	500,000
Runoff and AutoOne business
Discontinued Operations
Loss on sale of discontinued operation, net of tax	(91,000,000)	(18,200,000)	(91,000,000)	(18,200,000)
Net assets held for sale						25,000,000
Revenues
Net written premiums	(1,200,000)	12,700,000	600,000	47,200,000
Earned premiums	(400,000)	15,700,000	10,000,000	55,300,000
Net other revenues		100,000		1,600,000
Total revenues	(400,000)	15,800,000	10,000,000	56,900,000
Expenses
Loss and loss adjustment expenses	27,700,000	12,900,000	48,400,000	37,700,000
Policy acquisition expenses	(800,000)	1,900,000	(1,300,000)	5,400,000
Other underwriting expenses	(1,100,000)	5,200,000	1,100,000	16,600,000
Total expenses	25,800,000	20,000,000	48,200,000	59,700,000
Pre-tax loss	(26,200,000)	(4,200,000)	(38,200,000)	(2,800,000)
Income tax benefit	15,800,000	2,500,000	24,800,000	1,500,000
Loss from discontinued operations, net of tax	(15,800,000)	(2,500,000)	(24,800,000)	(1,500,000)
Reinsurance recoverable on paid losses	17,600,000		17,600,000			0
Reinsurance recoverable on unpaid losses	1,956,600,000		1,956,600,000			0
Net loss attributable to OneBeacon's common shareholders	$ (106,800,000)	$ (20,700,000)	$ (115,800,000)	$ (19,700,000)