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Summary of Significant Accounting Policies Revenue recognition (Details) (White Mountains Advisors LLC, USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
White Mountains Advisors LLC |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Investment Advisory, Management and Administrative Fees | $ 18.4 us-gaap_InvestmentAdvisoryManagementAndAdministrativeFees / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember | $ 16.6 us-gaap_InvestmentAdvisoryManagementAndAdministrativeFees / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember | $ 15.9 us-gaap_InvestmentAdvisoryManagementAndAdministrativeFees / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember

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- Definition Amount of revenue from providing investment advisory, asset management and administrative services, including, but not limited to, distribution and shareholder services, accounting, tax, legal, regulatory filing, share registration, and shareholder correspondence activities. + References No definition available. + Details
Name: | us-gaap_InvestmentAdvisoryManagementAndAdministrativeFees
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
Name: | us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration