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v2.4.1.9

Significant Transactions (Dispositions) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended | 3 Months Ended | 0 Months Ended |
--- | --- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2014 | Dec. 23, 2014 | Oct. 31, 2014
Significant Transactions |  |  |  |  |  |
(Loss) gain on sale of discontinued operations, net of tax | $ (1.6)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax | $ 46.6 us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax | $ (91.0)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax |  |  |
Gain (Loss) on Disposition of Stock in Subsidiary | 0.7 us-gaap_GainOrLossOnSaleOfStockInSubsidiary | 21.3 us-gaap_GainOrLossOnSaleOfStockInSubsidiary | 0 us-gaap_GainOrLossOnSaleOfStockInSubsidiary |  |  |
Premiums Written, Net | 2,121.5 us-gaap_PremiumsWrittenNet |  |  |  |  |
OneBeacon Runoff |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |
Surplus notes |  |  |  |  | 101.0 us-gaap_SurplusNotes / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember |
Ashmere [Member] | WM Solutions |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |
Excess amount over statutory surplus |  |  |  |  |  | 6.0 wtm_Excessamountoverstatutorysurplus / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_AshmereMember / dei_LegalEntityAxis = wtm_WmSolutionsMember
Gain (Loss) on Disposition of Stock in Subsidiary | 1.0 us-gaap_GainOrLossOnSaleOfStockInSubsidiary / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_AshmereMember / dei_LegalEntityAxis = wtm_WmSolutionsMember |  |  |  |  |
Citation [Member] | WM Solutions |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |
Gain (Loss) on Disposition of Stock in Subsidiary |  |  |  | 0.7 us-gaap_GainOrLossOnSaleOfStockInSubsidiary / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_CitationMember / dei_LegalEntityAxis = wtm_WmSolutionsMember |  |
Essentia | OneBeacon |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |
Gain (Loss) on Disposition of Stock in Subsidiary |  | 23.0 us-gaap_GainOrLossOnSaleOfStockInSubsidiary / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_EssentiaMember / dei_LegalEntityAxis = wtm_OnebeaconMember |  |  |  |
Gain (Loss) on Disposition of Stock in Subsidiary, After Tax |  | 15.0 wtm_GainLossonDispositionofStockinSubsidiaryAfterTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_EssentiaMember / dei_LegalEntityAxis = wtm_OnebeaconMember |  |  |  |
Underwriting Agreement [Member] | Essentia | OneBeacon |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |
Premiums Written, Net |  |  | 179.7 us-gaap_PremiumsWrittenNet / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_EssentiaMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_TypeOfArrangementAxis = wtm_UnderwritingAgreementMember |  |  |
Premiums Written, Percentage of Total |  |  | 8.00%wtm_PremiumsWrittenPercentageOfTotal / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_EssentiaMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_TypeOfArrangementAxis = wtm_UnderwritingAgreementMember |  |  |
Fair Value [Member] | Other long-term investments | OneBeacon Runoff |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |
Surplus notes |  |  |  |  | 64.9 us-gaap_SurplusNotes / us-gaap_FairValueByAssetClassAxis = wtm_FairValueMember / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember |
Discontinued Operations |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |
(Loss) gain on sale of discontinued operations, net of tax | (1.6)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember | 46.6 us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember | (91.0)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Discontinued Operations | OneBeacon Runoff |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |
Surplus notes | 101.0 us-gaap_SurplusNotes / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |  |  |
(Loss) gain on sale of discontinued operations, net of tax | (35.9)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  | 91.5 us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |
Discontinued Operations | Surplus Note | OneBeacon Runoff |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |
(Loss) gain on sale of discontinued operations, net of tax | (23.5)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / invest_InvestmentAxis = wtm_SurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |  |  |
Discontinued Operations | Fair Value [Member] | OneBeacon Runoff |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |
Surplus notes | 65.1 us-gaap_SurplusNotes / us-gaap_FairValueByAssetClassAxis = wtm_FairValueMember / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |  |  |
Surplus Note | Discontinued Operations | OneBeacon Runoff |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |
(Loss) gain on sale of discontinued operations, net of tax | $ 36.1 us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_InvestmentTypeAxis = wtm_SurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |  | $ 36.1 us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_InvestmentTypeAxis = wtm_SurplusNoteMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |

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- Definition Amount of gain (loss), after tax expense or benefit and not previously recognized, resulting from the sale of a business component. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e957-107759 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760 + Details
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