Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	$ 69.2	[1]	$ 98.3	[1]
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	135.7		222.1
Derivative, Collateral, Right to Reclaim Cash	0		45.3
Derivative, Collateral, Right to Reclaim Securities	0.40		0
Derivative Asset, Fair Value, Amount Offset Against Collateral	69.6		143.6
Excess collateral provided to counterparty in cash	44.1		81.1
Excess collateral providedto counterparty in financial instruments	22.8		28.0
Derivative, Collateral, Obligation to Return Cash	0.8		30.6
Forward contracts
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	(0.1)		(0.1)
Credit risk on derivative instruments
S&P Credit Ratings	21
A | Citigroup OTC [Member] | White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	19.4		19.9
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	21.7		50.7
Derivative, Collateral, Right to Reclaim Cash	0		0
Derivative, Collateral, Right to Reclaim Securities	0		0
Derivative Asset, Fair Value, Amount Offset Against Collateral	19.4		19.9
Excess collateral provided to counterparty in cash	2.3		30.8
Excess collateral providedto counterparty in financial instruments	0		0
Derivative, Collateral, Obligation to Return Cash	0		0
A | Royal Bank of Scotland [Member] | White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	11.3		33.6
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	11.3		33.6
Derivative, Collateral, Right to Reclaim Cash	0		0
Derivative, Collateral, Right to Reclaim Securities	0		0
Derivative Asset, Fair Value, Amount Offset Against Collateral	11.3		33.6
Excess collateral provided to counterparty in cash	0		0
Excess collateral providedto counterparty in financial instruments	0		0
Derivative, Collateral, Obligation to Return Cash	0		0
A | Barclays [Member] | White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	1.4
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	1.4
Derivative, Collateral, Right to Reclaim Cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1.4
Excess collateral provided to counterparty in cash	0
Excess collateral providedto counterparty in financial instruments	0
Derivative, Collateral, Obligation to Return Cash	0
A | Bank of America [Member] | White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	27.2		78.5
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	27.2		47.9
Derivative, Collateral, Right to Reclaim Cash	0		0
Derivative, Collateral, Right to Reclaim Securities	0		0
Derivative Asset, Fair Value, Amount Offset Against Collateral	27.2		78.5
Excess collateral provided to counterparty in cash	0		0
Excess collateral providedto counterparty in financial instruments	0		0
Derivative, Collateral, Obligation to Return Cash	0		30.6
A | Citigroup Exchange Traded [Member] | White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	1.2		(21.5)
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	21.0		13.6
Derivative, Collateral, Right to Reclaim Cash	0		21.5
Derivative, Collateral, Right to Reclaim Securities	0		0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1.2		0
Excess collateral provided to counterparty in cash	19.8		13.6
Excess collateral providedto counterparty in financial instruments	0		0
Derivative, Collateral, Obligation to Return Cash	0		0
BBB | Nomura [Member] | White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	(0.4)		(0.9)
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	22.0		28.8
Derivative, Collateral, Right to Reclaim Cash	0		0.9
Derivative, Collateral, Right to Reclaim Securities	0.40		0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0		0
Excess collateral provided to counterparty in cash	0		0.8
Excess collateral providedto counterparty in financial instruments	22.8		28.0
Derivative, Collateral, Obligation to Return Cash	0.8		0
Standard & Poor's, A+ Rating [Member] | J P Morgan Chase [Member] | White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	9.1		(22.8)
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	31.1		32.8
Derivative, Collateral, Right to Reclaim Cash	0		22.8
Derivative, Collateral, Right to Reclaim Securities	0		0
Derivative Asset, Fair Value, Amount Offset Against Collateral	9.1		0
Excess collateral provided to counterparty in cash	22.0		32.8
Excess collateral providedto counterparty in financial instruments	0		0
Derivative, Collateral, Obligation to Return Cash	0		0
Standard & Poor's, A+ Rating [Member] | Barclays [Member] | White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets			11.6
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments			11.6
Derivative, Collateral, Right to Reclaim Cash			0
Derivative, Collateral, Right to Reclaim Securities			0
Derivative Asset, Fair Value, Amount Offset Against Collateral			11.6
Excess collateral provided to counterparty in cash			0
Excess collateral providedto counterparty in financial instruments			0
Derivative, Collateral, Obligation to Return Cash			0
Standard & Poor's, A- Rating [Member] | Goldman Sachs [Member] | White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets			(0.1)
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments			3.1
Derivative, Collateral, Right to Reclaim Cash			0.1
Derivative, Collateral, Right to Reclaim Securities			0
Derivative Asset, Fair Value, Amount Offset Against Collateral			0
Excess collateral provided to counterparty in cash			3.1
Excess collateral providedto counterparty in financial instruments			0
Derivative, Collateral, Obligation to Return Cash			$ 0

[1] All derivative instruments held by WM Life Re are subject to master netting arrangements.