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Derivatives (Details) ¥ in Billions, $ in Billions | Sep. 30, 2015 USD ($) | Sep. 30, 2015 JPY (¥) | Dec. 31, 2014 USD ($) | Dec. 31, 2014 JPY (¥)
--- | --- | --- | --- | ---
Collective account values of underlying variable annuities as a percentage of the guarantee value |  |  | 113.00% | 113.00%
White Mountains Life Re |  |  |  |
Notional amount | $ 0.6 | ¥ 76.3 | $ 1.1 | ¥ 134.2
Collective account values of underlying variable annuities as a percentage of the guarantee value | 107.00% | 107.00% |  |

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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Namespace Prefix: | invest_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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- Definition Represents the collective account values of underlying variable annuities as a percentage of the guarantee value of the associated variable annuity reinsurance liability. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | dei_LegalEntityAxis=wtm_WhiteMountainsLifeReMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |