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Investment Securities (Fair Value Measurement by Level) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014 | Mar. 31, 2014 | Dec. 31, 2013
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 5,957.2 us-gaap_InvestmentsFairValueDisclosure | $ 5,991.4 us-gaap_InvestmentsFairValueDisclosure | $ 6,736.7 us-gaap_InvestmentsFairValueDisclosure | $ 6,766.5 us-gaap_InvestmentsFairValueDisclosure
Other Investments | 402.6 us-gaap_OtherInvestments | 407.0 us-gaap_OtherInvestments |  |
Short-term investments, at amortized cost (which approximates fair value) | 929.2 us-gaap_ShortTermInvestments | 871.7 us-gaap_ShortTermInvestments |  |
Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 805.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 843.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 1,565.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 1,376.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 4,594.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 4,638.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 4,727.5 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 4,982.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2,215.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 2,311.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |
Mortgage-backed and asset-backed securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2,098.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 1,840.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  |
Mortgage-backed and asset-backed securities | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2,030.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 1,840.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  |
Mortgage-backed and asset-backed securities | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 67.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  |
Fixed maturity investments. | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 139.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 76.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 112.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 93.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Common equity securities | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 32.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 40.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 47.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 46.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Convertible fixed maturity investments | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 3.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 8.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 6.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 6.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Other long-term investments | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 382.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 385.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 277.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 262.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Fair value measured on a recurring basis |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 6,886.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 6,863.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Fair value measured on a recurring basis | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 1,731.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 1,712.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Fair value measured on a recurring basis | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 4,596.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 4,640.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Fair value measured on a recurring basis | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 558.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 510.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Fair value measured on a recurring basis | U.S. Government and agency obligations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 165.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 188.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  |
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 110.8 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 134.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  |
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 54.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 54.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  |
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  |
Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2,215.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 2,311.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2,215.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 2,305.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 5.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 538.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 575.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 538.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 575.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 252.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_IndustrialMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 225.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_IndustrialMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_IndustrialMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 252.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_IndustrialMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 219.8 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_IndustrialMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_IndustrialMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 5.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_IndustrialMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 398.5 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_FinancialsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 431.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_FinancialsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_FinancialsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_FinancialsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments |  |  |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Technology |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Materials |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Other: |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Municipal obligations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Municipal obligations | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  |
Fair value measured on a recurring basis | Municipal obligations | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 97.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 83.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  |
Fair value measured on a recurring basis | Municipal obligations | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  |
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2,098.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 1,840.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  |
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  |
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2,030.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 1,840.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  |
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 67.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  |
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 108.8 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_ForeignGovernmentExcludingUnitedStatesDebtSecuritiesMember | 275.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_ForeignGovernmentExcludingUnitedStatesDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 18.5 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_ForeignGovernmentExcludingUnitedStatesDebtSecuritiesMember | 21.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_ForeignGovernmentExcludingUnitedStatesDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 90.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_ForeignGovernmentExcludingUnitedStatesDebtSecuritiesMember | 253.8 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_ForeignGovernmentExcludingUnitedStatesDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_ForeignGovernmentExcludingUnitedStatesDebtSecuritiesMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_ForeignGovernmentExcludingUnitedStatesDebtSecuritiesMember |  |
Fair value measured on a recurring basis | Preferred Stock |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 86.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember | 85.8 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember |  |
Fair value measured on a recurring basis | Preferred Stock | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember |  |
Fair value measured on a recurring basis | Preferred Stock | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 14.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember | 14.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember |  |
Fair value measured on a recurring basis | Preferred Stock | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 71.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember | 71.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember |  |
Fair value measured on a recurring basis | Fixed maturity investments. |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 4,771.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 4,784.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember |  |
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 129.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 155.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember |  |
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 4,503.5 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 4,552.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember |  |
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 139.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 76.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember |  |
Fair value measured on a recurring basis | Short-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 929.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 871.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember |  |
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 926.8 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 868.8 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember |  |
Short-term investments, at amortized cost (which approximates fair value) | 929.2 us-gaap_ShortTermInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 871.7 us-gaap_ShortTermInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 674.0 us-gaap_ShortTermInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 635.9 us-gaap_ShortTermInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 2.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember |  |
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember |  |
Fair value measured on a recurring basis | Common equity securities | Consumer |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 170.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 161.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  |
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 170.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 161.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  |
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 0.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  |
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_ConsumerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  |
Fair value measured on a recurring basis | Common equity securities | Industrial |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 70.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_IndustrialMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 67.5 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_IndustrialMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  |
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 70.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_IndustrialMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 67.5 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_IndustrialMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  |
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_IndustrialMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  |  |
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_IndustrialMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  |  |
Fair value measured on a recurring basis | Common equity securities | Financials |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 226.8 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_FinancialsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 237.5 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / invest_InvestmentSectorAxis = wtm_FinancialsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  |
Fair value measured on a recurring basis | Common equity securities | Financials | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Financials | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Communications [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Health Care [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Energy |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Energy | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Energy | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Energy | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Technology |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Technology | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Utilities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Utilities | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Materials |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Materials | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Other: |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
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