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Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	Mar. 31, 2013 Rating		Dec. 31, 2012
Credit risk on derivative instruments
Collateral provided - Financial Instruments	$ 75.8	[1]	$ 98.3	[1]
Derivative, Collateral, Right to Reclaim Securities	25.6		28.1
Collateral held - Financial Instruments	0		0
Collateral provided - Cash	89.5		126.3
Collateral held - Cash	10.8		30.6
Net amount of exposure after effect of collateral held	180.1		222.1
S and P Credit Ratings	21
A | Bank of America
Credit risk on derivative instruments
Collateral provided - Financial Instruments	60.0		78.5
Derivative, Collateral, Right to Reclaim Securities	0		0
Collateral held - Financial Instruments	0		0
Collateral provided - Cash	0		0
Collateral held - Cash	10.8		30.6
Net amount of exposure after effect of collateral held	49.2		47.9
A | Royal Bank of Scotland
Credit risk on derivative instruments
Collateral provided - Financial Instruments	26.4		33.6
Derivative, Collateral, Right to Reclaim Securities	0		0
Collateral held - Financial Instruments	0		0
Collateral provided - Cash	0		0
Collateral held - Cash	0		0
Net amount of exposure after effect of collateral held	26.4		33.6
A | Citigroup - OTC
Credit risk on derivative instruments
Collateral provided - Financial Instruments	(5.3)		19.9
Derivative, Collateral, Right to Reclaim Securities	0		0
Collateral held - Financial Instruments	0		0
Collateral provided - Cash	25.3		30.8
Collateral held - Cash	0		0
Net amount of exposure after effect of collateral held	20.0		50.7
A | Citigroup - Exchange Traded
Credit risk on derivative instruments
Collateral provided - Financial Instruments	(8.7)		(21.5)
Derivative, Collateral, Right to Reclaim Securities	0		0
Collateral held - Financial Instruments	0		0
Collateral provided - Cash	24.7		35.0
Collateral held - Cash	0		0
Net amount of exposure after effect of collateral held	16.0		13.5
A | Barclays
Credit risk on derivative instruments
Collateral provided - Financial Instruments	6.5		11.6
Derivative, Collateral, Right to Reclaim Securities	0		0
Collateral held - Financial Instruments	0		0
Collateral provided - Cash	0		0
Collateral held - Cash	0		0
Net amount of exposure after effect of collateral held	6.5		11.6
A | JP Morgan
Credit risk on derivative instruments
Collateral provided - Financial Instruments	3.1		(22.8)
Derivative, Collateral, Right to Reclaim Securities	0		0
Collateral held - Financial Instruments	0		0
Collateral provided - Cash	31.3		55.6
Collateral held - Cash	0		0
Net amount of exposure after effect of collateral held	34.4		32.8
Standard & Poor's, BBB Plus Rating | Nomura
Credit risk on derivative instruments
Collateral provided - Financial Instruments	(6.2)		(0.9)
Derivative, Collateral, Right to Reclaim Securities	25.6		28.1
Collateral held - Financial Instruments	0		0
Collateral provided - Cash	8.2		1.7
Collateral held - Cash	0		0
Net amount of exposure after effect of collateral held	27.6		28.9
A- | Goldman Sachs
Credit risk on derivative instruments
Collateral provided - Financial Instruments	0		(0.1)
Derivative, Collateral, Right to Reclaim Securities	0		0
Collateral held - Financial Instruments	0		0
Collateral provided - Cash	0		3.2
Collateral held - Cash	0		0
Net amount of exposure after effect of collateral held	$ 0		$ 3.1
[1] All derivative instruments held by WM Life Re are subject to master netting arrangements.

X

- Definition

The amount of the obligation to return cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 8

-Subparagraph (b)

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- Definition

The amount of the right to reclaim cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 8

-Subparagraph (b)

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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 in accordance with the entity's accounting policy were offset against collateral under a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 8

-Subparagraph (a)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Derivative, Collateral, Obligation to Return Securities

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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.

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

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