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Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
White Mountains Life Re
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	$ 212.2		$ 305.2
Citigroup | White Mountains Life Re
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	54.9	[1]
JP Morgan | White Mountains Life Re
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	30.9	[1]
Nomura | White Mountains Life Re
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	26.9	[1]
Barclays | White Mountains Life Re
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	12.3
Goldman Sachs | White Mountains Life Re
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	3.1
Bank of America [Member] | White Mountains Life Re
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	49.6
Royal Bank of Scotland [Member] | White Mountains Life Re
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	34.5
Forward contracts
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	31.2
S&P Credit Ratings	21
Forward contracts | JP Morgan
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	1.9
Forward contracts | Goldman Sachs
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	$ 0.4
[1] Collateral provided (held) calculated under master netting agreement.

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- Definition

The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that were not covered by collateral.

+ References

No definition available.

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- Definition

Represents the S&P creditworthiness heirarchy ratings used by the entity.

+ References

No definition available.

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