Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) - White Mountains Life Re $ in Thousands	Jun. 30, 2016 USD ($) Rating	Dec. 31, 2015 USD ($)
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 0	$ 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
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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