Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	Sep. 30, 2014 Rating	Dec. 31, 2013
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 65.3	$ 69.2
Collertal provided to counterparty in cash	0.3	0
Collateral provided to counterparty in financial instruments	0	0.40
Net amount of exposure after effect of collateral	65.6	69.6
Excess collateral provided to counterparty in cash	23.4	44.1
Derivative, Collateral, Right to Reclaim Securities	10.4	22.8
Counter-party collateral held by WMLife Re - Cash	0	(0.8)
Net amount of exposure to counter-party	99.4	135.7
S and P Credit Ratings	21
Standard & Poor's, A Rating | Bank of America [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	16.5	27.2
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	16.5	27.2
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	16.5	27.2
Standard & Poor's, A Rating | Barclays
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	0.5	1.4
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	0.5	1.4
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	0.5	1.4
Standard & Poor's, A Rating | Royal Bank of Scotland
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments		11.3
Collertal provided to counterparty in cash		0
Collateral provided to counterparty in financial instruments		0
Net amount of exposure after effect of collateral		11.3
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		11.3
Standard & Poor's, A Rating | Citigroup - OTC
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	15.5	19.4
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	15.5	19.4
Excess collateral provided to counterparty in cash	6.9	2.3
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	22.4	21.7
Standard & Poor's, A Rating | Citigroup Exchange Traded [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	4.3	1.2
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	4.3	1.2
Excess collateral provided to counterparty in cash	13.9	19.8
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	18.2	21.0
Standard & Poor's, A plus | JP Morgan
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	21.7	9.1
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	9.1
Excess collateral provided to counterparty in cash	0	22.0
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	31.1
Standard & Poor's, BBB+ Rating [Member] | Nomura [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	(0.3)	(0.4)
Collertal provided to counterparty in cash	0.3	0
Collateral provided to counterparty in financial instruments	0	0.40
Net amount of exposure after effect of collateral	0	0
Excess collateral provided to counterparty in cash	2.6	0
Derivative, Collateral, Right to Reclaim Securities	10.4	22.8
Counter-party collateral held by WMLife Re - Cash	0	(0.8)
Net amount of exposure to counter-party	13.0	22.0
Standard & Poor's, A- Rating | Royal Bank of Scotland
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	7.1
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	7.1
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	$ 7.1