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Derivatives (Forward Contract Collateral) (Details) (Forward Contracts, USD $) In Millions, unless otherwise specified | Mar. 31, 2013 Rating | Dec. 31, 2012
--- | --- | ---
Derivative [Line Items] |  |
Net amount of exposure after effect of collateral held | $ 79.3 | $ 31.2
Uncollaterailized balance of forward currency derivatives | (0.2) | (0.1)
S and P Credit Ratings | 21 |
A | Barclays |  |
Derivative [Line Items] |  |
Net amount of exposure after effect of collateral held | 24.7 | 7.7
Uncollaterailized balance of forward currency derivatives | 0.1 | (0.1)
A | Deutsche Bank |  |
Derivative [Line Items] |  |
Net amount of exposure after effect of collateral held | 13.8 | 11.1
Uncollaterailized balance of forward currency derivatives | (0.2) | 0
A | JP Morgan |  |
Derivative [Line Items] |  |
Net amount of exposure after effect of collateral held | 22.1 | 1.9
Uncollaterailized balance of forward currency derivatives | 0.1 | 0
A- | Goldman Sachs |  |
Derivative [Line Items] |  |
Net amount of exposure after effect of collateral held | 1.1 | 0.4
Uncollaterailized balance of forward currency derivatives | 0 | 0
AA- | HSBC |  |
Derivative [Line Items] |  |
Net amount of exposure after effect of collateral held | 0.1 | 10.1
Uncollaterailized balance of forward currency derivatives | 0 | 0
AA- | Royal Bank of Canada |  |
Derivative [Line Items] |  |
Net amount of exposure after effect of collateral held | 17.5 | 0
Uncollaterailized balance of forward currency derivatives | $ (0.2) | $ 0

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- Details
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- Definition The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that were not covered by collateral. + References No definition available. + Details
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Namespace Prefix: | wtm_
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- Definition Represents the S&P creditworthiness heirarchy ratings used by the entity. + References No definition available. + Details
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- Definition Uncollaterailized Balance of Forward Currency Derivatives + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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