Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) - White Mountains Life Re $ in Thousands	Dec. 31, 2015 USD ($) Rating
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 20,100
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	20,100
Excess collateral provided to counterparty in cash	5,800
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	6,900
Net amount of exposure to counter-party	$ 19,000
S and P Credit Ratings \| Rating	23
Standard & Poor's, A Rating [Member] \| Bank of America [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 700
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	700
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	700
Standard & Poor's, A Rating [Member] \| Citigroup - OTC
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	9,900
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	9,900
Excess collateral provided to counterparty in cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	1,400
Net amount of exposure to counter-party	8,500
Standard & Poor's, A Rating [Member] \| Citigroup Exchange Traded [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	1,000
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	1,000
Excess collateral provided to counterparty in cash	5,800
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	0
Net amount of exposure to counter-party	6,800
Standard & Poor's, A plus \| JP Morgan
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	8,500
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	8,500
Excess collateral provided to counterparty in cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	5,500
Net amount of exposure to counter-party	$ 3,000

Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

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)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=d3e41678-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Publisher FASB
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+ References

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