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Pension Plan (Details) | 9 Months Ended
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Sep. 30, 2015
Compensation Related Costs [Abstract] |
Months of Plan Assets to be Distributed | 12 months

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- References No definition available. + Details
Name: | us-gaap_CompensationRelatedCostsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Months of Plan Assets to be Distributed + References No definition available. + Details
Name: | wtm_MonthsofPlanAssetstobeDistributed
Namespace Prefix: | wtm_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration