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Loss and Loss Adjustment Expense Reserves Details (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2014 | Mar. 31, 2013
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Net favorable and unfavorable loss reserve development | $ (11.2) | $ (1.4)
Accretion of fair value adjustment to net loss and LAE reserves | 0.2 | 1.2
Adjustment to loss and loss adjustment expense reserves, before tax | 4.5 |
Sirius Group |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Net favorable and unfavorable loss reserve development | (9.8) | (4.3)
OneBeacon |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Net favorable and unfavorable loss reserve development | $ (1.4) | $ (2.9)

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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 + Details
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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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- 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_
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