Third Party Reinsurance (Details) (USD $)						1 Months Ended	3 Months Ended	6 Months Ended	122 Months Ended	1 Months Ended	3 Months Ended	6 Months Ended		3 Months Ended	6 Months Ended			3 Months Ended	6 Months Ended	3 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2011 OneBeacon	May 31, 2001 OneBeacon	Jun. 30, 2011 OneBeacon National Indemnity Company and General Reinsurance Corporation	May 31, 2001 OneBeacon National Indemnity Company NICO Cover	Jun. 30, 2011 OneBeacon National Indemnity Company NICO Cover	Jun. 30, 2011 OneBeacon National Indemnity Company NICO Cover	Jun. 30, 2011 OneBeacon National Indemnity Company NICO Cover	May 31, 2001 OneBeacon General Reinsurance Corporation GRC Cover	Jun. 30, 2011 OneBeacon General Reinsurance Corporation GRC Cover	Jun. 30, 2011 OneBeacon General Reinsurance Corporation GRC Cover	Jun. 30, 2011 OneBeacon Tower Insurance Company	Jun. 30, 2011 OneBeacon Tokio Marine and Nichido Fire	Jun. 30, 2011 OneBeacon Tokio Marine and Nichido Fire	Jun. 30, 2011 OneBeacon Hanover Insurance Company	Jun. 30, 2011 OneBeacon Munich Reinsurance America	Jun. 30, 2010 OneBeacon Northeast homeowners business	Jun. 30, 2010 OneBeacon Northeast homeowners business	Jun. 30, 2011 OneBeacon Property catastrophe reinsurance program	Jun. 30, 2011 White Mountains Re Segment	Jun. 30, 2011 White Mountains Re Segment General Reinsurance Corporation	Jun. 30, 2011 White Mountains Re Segment Lloyds of London	Jun. 30, 2011 White Mountains Re Segment Olympus	Jun. 30, 2011 White Mountains Re Segment Swiss Re Group	Jun. 30, 2011 White Mountains Re Segment Michigan Catastrophic Claims Association
Third party reinsurance
Reinsurance recoverable on paid losses	$ 44,500,000	$ 63,100,000	$ 16,200,000																		$ 28,400,000
Reinsurance recoverable on unpaid losses	2,193,100,000	2,344,000,000	1,995,800,000																		367,200,000
Purchase accounting adjustments			169,900,000
Percentage of total paid and unpaid reinsurance recoverables (as a percent)					74.00%								3.00%	3.00%	3.00%	6.00%	2.00%					11.00%	11.00%	11.00%	10.00%	4.00%
Reinsurance recoverables					1,491,400,000								62,300,000	64,200,000	64,200,000	113,600,000	32,500,000					43,300,000	43,700,000	44,800,000	39,500,000	15,200,000
Third party recoverables which reinsurer would pay if they are unable to collect from third party reinsurers							193,500,000	193,500,000	193,500,000
Reinsurance recoverable from various third party reinsurers, guaranteed by the third party reinsurer under reinsurance agreement														37,100,000	37,100,000
The percentage ceded under quota share reinsurance agreement (in percent)														100.00%	100.00%			30.00%
Number of reinsurance contracts the seller caused OneBeacon to purchase immediately prior to the entity's acquisition of OneBeacon				2
Maximum cover under reinsurance contract						2,500,000,000	2,500,000,000	2,500,000,000		570,000,000
Adverse loss reserve development on losses occurring in years 2000 and prior										400,000,000
Reserves ceded as of the date of the OneBeacon acquisition										170,000,000
Utilized coverage from NICO									2,200,000,000
Reinsurance receivables paid by the reinsurer									1,300,000,000		13,300,000	47,100,000
Percentage of utilized coverage from NICO that relates to uncollectible Third Party Recoverables (as a percent)									8.00%
Utilized coverage from NICO that relates to uncollectible Third Party Recoverables									182,000,000
Maximum amount of loss on a single catastrophe retained, first layer																				50,000,000
Amount of loss on a single catastrophe reinsured, second layer																				175,000,000
First retention percentage of loss on a single catastrophe reinsured, second layer (as a percent)																				26.00%
Amount of loss on a single catastrophe reinsured, first retention slab for second layer, low end of range																				50,000,000
Amount of loss on a single catastrophe reinsured, first retention slab for second layer, high end of range																				100,000,000
Second retention percentage of loss on a single catastrophe reinsured, second layer (as a percent)																				10.00%
Amount of loss on a single catastrophe reinsured, second retention slab for second layer, low end of range																				100,000,000
Amount of loss on a single catastrophe reinsured, second retention slab for second layer, high end of range																				175,000,000
Minimum amount of loss on a single catastrophe retained, third layer																				225,000,000
Premiums ceded under quota share agreement																		$ 14,000,000	$ 25,600,000
Percentage collateralized (as a percent)																						2.00%	3.00%	100.00%	4.00%
Number of ratings in A.M. Best ratings	15