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Investment Securities (Other Long-Term Investments) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
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Investment securities |  |  |
Other Investments | $ 385.0 us-gaap_OtherInvestments | $ 262.3 us-gaap_OtherInvestments |
Other long-term investments |  |  |
Investment securities |  |  |
Carrying value of investment accounted for using the equity method | 23.2 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember |  |
Limited Liability Company and Private Equity Securities |  |  |
Investment securities |  |  |
Other Investments | 69.7 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_LimitedliabilitycompaniesandprivateequitysecuritiesMember | 20.3 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_LimitedliabilitycompaniesandprivateequitysecuritiesMember |
Surplus Note |  |  |
Investment securities |  |  |
Other Investments | 65.1 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_SurplusNoteMember | 0 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_SurplusNoteMember |
Total hedge and private equity funds included in other long-term investments |  |  |
Investment securities |  |  |
Other Investments | 242.9 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_HedgeFundsAndPrivateEquityFundsMember | 239.0 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_HedgeFundsAndPrivateEquityFundsMember |
Other Debt Obligations [Member] |  |  |
Investment securities |  |  |
Other Investments | 7.3 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = us-gaap_OtherDebtSecuritiesMember | 3.1 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = us-gaap_OtherDebtSecuritiesMember |
Forward contracts |  |  |
Investment securities |  |  |
Other Investments | 0 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = us-gaap_ForwardContractsMember | (0.1)us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = us-gaap_ForwardContractsMember |
Fair value measured on a recurring basis | Level 3 Inputs | Other long-term investments |  |  |
Investment securities |  |  |
Carrying value of investment accounted for using the equity method | $ 23.2 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | $ 26.6 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | $ 35.0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember

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- Definition Fair value portion of investments accounted under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6903542&loc=d3e32014-111567 + Details
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- Definition Other investments not otherwise specified in the taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 12 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph g -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(f,g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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