Loss and Loss Adjustment Expense Reserves (Details) (USD $)	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		2 Months Ended		5 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011 OneBeacon Segment	Jun. 30, 2010 OneBeacon Segment	Jun. 30, 2011 OneBeacon Segment	Jun. 30, 2010 OneBeacon Segment	Jun. 03, 2011 White Mountains Re Segment	Jun. 03, 2010 White Mountains Re Segment	Jun. 03, 2011 White Mountains Re Segment	Jun. 03, 2010 White Mountains Re Segment
Loss and Loss Adjustment Expense Reserves
Gross beginning balance	$ 5,759,800,000	$ 6,660,700,000	$ 5,736,800,000	$ 6,379,200,000
Less beginning reinsurance recoverable on unpaid losses	(2,304,200,000)	(2,899,400,000)	(2,344,000,000)	(2,771,500,000)
Net loss and LAE reserves	3,455,600,000	3,761,300,000	3,392,800,000	3,607,700,000
Loss and LAE reserves acquired - Central National				17,600,000
Loss and LAE reserves sold - One Beacon Personal Lines		(231,000,000)		(231,000,000)
Loss and LAE incurred relating to Current year losses	302,700,000	428,300,000	692,500,000	991,700,000
Loss and LAE incurred relating to Prior year losses	(10,300,000)	(26,200,000)	(27,500,000)	(37,700,000)	(10,100,000)	(18,300,000)	(15,200,000)	(24,300,000)	(200,000)	(7,900,000)	(12,300,000)	(13,400,000)
Total incurred losses and LAE	292,400,000	402,100,000	665,000,000	954,000,000
Accretion of fair value adjustment to loss and LAE reserves	2,100,000	2,100,000	4,200,000	4,200,000
Foreign currency translation adjustment to loss and LAE reserves	10,100,000	(22,400,000)	31,200,000	(33,200,000)
Loss and LAE paid relating to Current year losses	(87,700,000)	(169,600,000)	(135,200,000)	(258,400,000)
Loss and LAE paid relating to Prior year losses	(223,200,000)	(292,700,000)	(508,700,000)	(611,100,000)
Total loss and LAE payments	(310,900,000)	(462,300,000)	(643,900,000)	(869,500,000)
Net ending balance	3,449,300,000	3,449,800,000	3,449,300,000	3,449,800,000
Plus ending reinsurance recoverable on unpaid losses	2,193,100,000	2,748,100,000	2,193,100,000	2,748,100,000
Gross ending balance	5,642,400,000	6,197,900,000	5,642,400,000	6,197,900,000
Net favorable loss reserve development	10,300,000	26,200,000	27,500,000	37,700,000	10,100,000	18,300,000	15,200,000	24,300,000	200,000	7,900,000	12,300,000	13,400,000
Net favorable loss reserve development due to 2010 Chilean earthquake			9,000,000
Net favorable loss reserve development due to 1999 aviation loss			9,000,000
Net unfavorable loss reserve development due to Asbestos and Environmental reserves additions			2,000,000
Net unfavorable loss reserve development from other business lines, mainly marine			3,700,000
Favorable loss reserve development of commercial and personal auto reserves								6,500,000
Pre-tax un-accreted adjustment			$ 16,800,000