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Investment Securities - Fair Value Measurements (Details) | 3 Months Ended
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Mar. 31, 2021
Investments, Debt and Equity Securities [Abstract] |
Percentage of investments recorded at fair value | 73.00%

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition Represents the percentage of the total investment portfolio recorded at fair value based upon quoted market prices or other observable inputs. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | num:percentItemType
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