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Investment Securities (Mortgage-backed, Asset-backed Securities) (Details) | 12 Months Ended
--- | ---
Dec. 31, 2012
Fixed rate CMBS |
Investment securities |
Average basis points of subordination (as a percent) | 25
Floating rate CMBS |
Investment securities |
Average basis points of subordination (as a percent) | 50
Maximum | Commercial |
Investment securities |
Percentage of underlying loans reported as non-performing | 1.00%

X
- Definition Reflects the percentage derived from dividing impaired loans on which interest has ceased to be accrued by total loans. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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

X
- Definition Basis points of subordination on average. + References No definition available. + Details
Name: | wtm_AverageBasisPointsOfSubordination
Namespace Prefix: | wtm_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration