Derivatives (Forward Contracts) (Details) In Millions, unless otherwise specified	12 Months Ended		12 Months Ended										12 Months Ended
	Dec. 31, 2012 Forward contracts USD ($) Rating	Dec. 31, 2012 Forward contracts SEK	Dec. 31, 2012 Forward contracts USD USD ($)	Dec. 31, 2012 Forward contracts SEK USD ($)	Dec. 31, 2012 Forward contracts EUR USD ($)	Dec. 31, 2012 Forward contracts GBP USD ($)	Dec. 31, 2012 Forward contracts Currency Translation USD ($)	Dec. 31, 2012 Forward contracts Deutsche Bank USD ($)	Dec. 31, 2012 Forward contracts Barclays Bank London USD ($)	Dec. 31, 2012 Forward contracts HSBC Bank plc USD ($)	Dec. 31, 2012 Forward contracts JP Morgan USD ($)	Dec. 31, 2012 Forward contracts Goldman Sachs USD ($)	Dec. 31, 2012 White Mountains Life Re USD ($)	Dec. 31, 2011 White Mountains Life Re USD ($)	Dec. 31, 2010 White Mountains Life Re USD ($)	Dec. 31, 2012 White Mountains Life Re JP Morgan USD ($)		Dec. 31, 2012 White Mountains Life Re Goldman Sachs USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	$ 31.2	202.8
S&P Credit Ratings	21
Changes in derivative instruments
Beginning of period	0												290.4
Purchases	0
Change in fair value of derivatives, included in other revenues	(0.3)		(0.2)	0	(0.1)	0	0						(339.0)	92.9	127.0
End of period	0.2
Change in fair value of derivatives, included in other revenues	(0.3)		(0.2)	0	(0.1)	0	0						(339.0)	92.9	127.0
Derivative, Fair Value, Net	(0.1)		0	0	(0.1)	0	0						98.3	290.4
Net exposure on fair value of OTC instruments	31.2							11.1	7.7	10.1	1.9	0.4	212.2	305.2		30.9	[1]	3.1
Uncollaterailized balance	$ (0.1)							$ 0	$ (0.1)	$ 0	$ 0	$ 0

[1] Collateral provided (held) calculated under master netting agreement.