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Loss and Loss Adjustment Expense Reserves Details (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Net favorable and unfavorable loss reserve development | $ 1.4 | $ (7.6)
Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims Prior Years Due To 2011 Japan Earthquake |  | 8.1
Accretion of fair value adjustment to net loss and LAE reserves | 1.2 | 7.1
Adjustment to loss and loss adjustment expense reserves, before tax | 4.5 |
One Beacon |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Net favorable and unfavorable loss reserve development | (2.9) | (1.9)
Sirius |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Net favorable and unfavorable loss reserve development | 4.3 | (5.7)
Scandinavian Reinsurance Company [Member] |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Accretion of fair value adjustment to net loss and LAE reserves increase |  | $ 5.0

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- Definition Amount of provision in the period for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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Balance Type: | credit
Period Type: | duration

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- Definition Represents the amount of accreted fair value adjustment to loss and LAE reserves related to purchase accounting. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Represents the amount of accreted fair value adjustment to loss and LAE reserves related to purchase accounting. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Represents the net favorable loss reserve development due to the reduction from the loss reserve that was a result of the Japan earthquake that occurred in 2011. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Details
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- Definition Represents the amount of pre-tax unaccreted fair value adjustment to loss and LAE reserves related to purchase accounting. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant