SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Rate at which reserves are discounted (as a percent)	3.50%		4.50%		5.00%
OneBeacon
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	$ 123.9		$ 123.5		$ 114.5
Reserves for Unpaid Claims and Claims Adjustment Expenses	1,000.0		3,358.6		3,295.5
Discount, if any, deducted in Column C	4.6	[1]	271.6	[1]	295.9	[1]
Unearned Premiums	573.8		528.0		627.5
Earned Premiums	1,132.0		1,012.2		1,181.1
Net investment income	53.6		71.4		96.6
Current Year	657.4		578.1		721.6
Prior Year	(7.4)		(29.8)		(36.0)
Amortization of deferred policy acquisition costs	249.4		221.2		252.1
Paid Claims and Claims Adjustment Expenses	565.1		523.2		654.8
Premiums written	1,179.2		1,062.7		1,167.7
Sirius Group
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	71.4		63.5		61.6
Reserves for Unpaid Claims and Claims Adjustment Expenses	2,168.9		2,343.7		2,441.3
Discount, if any, deducted in Column C	2.4	[2]	12.8	[2]	21.1	[2]
Unearned Premiums	350.2		319.0		311.2
Earned Premiums	931.6		912.3		847.9
Net investment income	65.0		89.9		90.5
Current Year	578.4		672.9		588.1
Prior Year	(34.5)		(46.9)		(57.1)
Amortization of deferred policy acquisition costs	180.8		181.0		167.5
Paid Claims and Claims Adjustment Expenses	741.2		642.0		437.0
Premiums written	$ 947.7		$ 915.7		$ 865.8

[1] The amounts shown represent OneBeacon’s discount on its long-term workers compensation loss and LAE reserves, as such liabilities constitute unpaid but settled claims under which the payment pattern and ultimate costs are fixed and determinable on an individual basis. OneBeacon discounts these reserves using a discount rate which is determined based on the facts and circumstances applicable at the time the claims are settled (3.5%, 4.5% and 5.0% at December 31, 2012, 2011 and 2010). Also the amounts shown include unamortized fair value adjustments to reserves for unpaid claims and claims adjustment expenses made in purchase accounting as a result of White Mountains’ purchase of OneBeacon for the years ended December 31, 2012, 2011 and 2010.

[2] The amount shown represents unamortized fair value adjustments to reserves for unpaid claims and claims adjustment expenses made in purchase accounting as a result of White Mountains’ purchase of Sirius International during 2004.