Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	Jun. 30, 2013 Rating		Dec. 31, 2012
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 141.3	[1]	$ 98.3	[1]
Collertal provided to counterparty in cash	9.1		45.3
Collateral provided to counterparty in financial instruments	0.60		0
Net amount of exposure after effect of collateral	151.0		143.6
Excess collateral provided to counterparty in cash	13.6		81.1
Derivative, Collateral, Right to Reclaim Securities	23.7		28.0
Counter-party collateral held by WMLife Re - Cash	30.4		30.6
Net amount of exposure to counter-party	157.9		222.1
S and P Credit Ratings	21
A | Bank of America [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	52.1		78.5
Collertal provided to counterparty in cash	0		0
Collateral provided to counterparty in financial instruments	0		0
Net amount of exposure after effect of collateral	52.1		78.5
Excess collateral provided to counterparty in cash	0		0
Derivative, Collateral, Right to Reclaim Securities	0		0
Counter-party collateral held by WMLife Re - Cash	6.3		30.6
Net amount of exposure to counter-party	45.8		47.9
A | Barclays
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	4.6
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	4.6
Excess collateral provided to counterparty in cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	0
Net amount of exposure to counter-party	4.6
A | JP Morgan
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	34.1
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	34.1
Excess collateral provided to counterparty in cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	5.9
Net amount of exposure to counter-party	28.2
A | Royal Bank of Scotland
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	21.7		33.6
Collertal provided to counterparty in cash	0		0
Collateral provided to counterparty in financial instruments	0		0
Net amount of exposure after effect of collateral	21.7		33.6
Excess collateral provided to counterparty in cash	0		0
Derivative, Collateral, Right to Reclaim Securities	0		0
Counter-party collateral held by WMLife Re - Cash	0		0
Net amount of exposure to counter-party	21.7		33.6
A | Citigroup - OTC
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	38.5		19.9
Collertal provided to counterparty in cash	0		0
Collateral provided to counterparty in financial instruments	0		0
Net amount of exposure after effect of collateral	38.5		19.9
Excess collateral provided to counterparty in cash	0		30.8
Derivative, Collateral, Right to Reclaim Securities	0		0
Counter-party collateral held by WMLife Re - Cash	18.2		0
Net amount of exposure to counter-party	20.3		50.7
A | Citigroup Exchange Traded [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	(3.4)		(21.5)
Collertal provided to counterparty in cash	3.4		21.5
Collateral provided to counterparty in financial instruments	0		0
Net amount of exposure after effect of collateral	0		0
Excess collateral provided to counterparty in cash	13.6		13.6
Derivative, Collateral, Right to Reclaim Securities	0		0
Counter-party collateral held by WMLife Re - Cash	0		0
Net amount of exposure to counter-party	13.6		13.6
A | Barclays
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments			11.6
Collertal provided to counterparty in cash			0
Collateral provided to counterparty in financial instruments			0
Net amount of exposure after effect of collateral			11.6
Excess collateral provided to counterparty in cash			0
Derivative, Collateral, Right to Reclaim Securities			0
Counter-party collateral held by WMLife Re - Cash			0
Net amount of exposure to counter-party			11.6
A | JP Morgan
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments			(22.8)
Collertal provided to counterparty in cash			22.8
Collateral provided to counterparty in financial instruments			0
Net amount of exposure after effect of collateral			0
Excess collateral provided to counterparty in cash			32.8
Derivative, Collateral, Right to Reclaim Securities			0
Counter-party collateral held by WMLife Re - Cash			0
Net amount of exposure to counter-party			32.8
Standard & Poor's, BBB+ Rating [Member] | Nomura [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	(6.3)		(0.9)
Collertal provided to counterparty in cash	5.7		0.9
Collateral provided to counterparty in financial instruments	0.60		0
Net amount of exposure after effect of collateral	0		0
Excess collateral provided to counterparty in cash	0		0.8
Derivative, Collateral, Right to Reclaim Securities	23.7		28.0
Counter-party collateral held by WMLife Re - Cash	0		0
Net amount of exposure to counter-party	23.7		28.8
A- | Goldman Sachs
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	0		(0.1)
Collertal provided to counterparty in cash	0		0.1
Collateral provided to counterparty in financial instruments	0		0
Net amount of exposure after effect of collateral	0		0
Excess collateral provided to counterparty in cash	0		3.1
Derivative, Collateral, Right to Reclaim Securities	0		0
Counter-party collateral held by WMLife Re - Cash	0		0
Net amount of exposure to counter-party	$ 0		$ 3.1

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