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Investment Securities (Fair Value Measurements) (Details) (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2017 | Dec. 31, 2016
Percentage of investments recorded at fair value | 93.00% | 94.00%
Maximum percentage of price difference provided by pricing services | 5.00% |
Maximum price difference provided by pricing services | $ 1.0 |

X
- Definition Represents the maximum price difference provided by the pricing services between the market price and the actual sale price of securities, on the basis of which outliers are considered. + References No definition available. + Details
Name: | wtm_MaximumDifferencesBetweenPricesProvidedByPricingServicesConsideredOutliers
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the maximum percentage of price difference provided by the pricing services between the market price and the actual sale price of securities, on the basis of which outliers are considered. + References No definition available. + Details
Name: | wtm_MaximumPercentageDifferencesBetweenPricesProvidedByPricingServicesConsideredOutliers
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- 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
Name: | wtm_PercentageOfInvestmentsRecordedAtFairValue
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration