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Goodwill and Other Intangible Assets - Narrative (Details) $ in Millions | 3 Months Ended
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Mar. 31, 2021 USD ($)
Goodwill and Intangible Assets Disclosure [Abstract] |
Loss on sale | $ 28.7

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

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- Definition Disposal Group, Held for Sale, Remeasurement Loss + References No definition available. + Details
Name: | wtm_DisposalGroupHeldForSaleRemeasurementLoss
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration