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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, 2015 | Mar. 31, 2014
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Net (favorable) and unfavorable loss reserve development | $ (1.4)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears | $ (11.2)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears
Sirius Group |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Net (favorable) and unfavorable loss reserve development | 0.5 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (9.8)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Star & Shield Insurance Exchange |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Net (favorable) and unfavorable loss reserve development | (0.1)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_StarShieldInsuranceExchangeMember |
OneBeacon |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Net (favorable) and unfavorable loss reserve development | $ (1.8)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | $ (1.4)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember

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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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- Details
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- Details
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