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v2.4.0.6

Retirement and Postretirement Plans (Details) | 12 Months Ended
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Dec. 31, 2011 Y
Retirement and Postretirement Plans |
Period of service after which plan gets vested (in years) | 5

X
- Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the period over which plan gets vested to the employee under the defined benefit plan. + References No definition available. + Details
Name: | wtm_DefinedBenefitPlanVestingPeriod
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration