XML 31 R70.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Third-Party Reinsurance (OneBeacon) (Details) (USD $) | 12 Months Ended | 3 Months Ended | 12 Months Ended | 3 Months Ended | 7 Months Ended | 12 Months Ended | 60 Months Ended |  |
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Dec. 31, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2014 | Dec. 31, 2020 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2020 | Dec. 31, 2012 | Dec. 31, 2011
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on unpaid losses | $ 483,900,000 us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims |  | $ 428,100,000 us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims | $ 483,900,000 us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims | $ 483,900,000 us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims |  |  |  |  | $ 429,100,000 us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims | $ 2,507,300,000 us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims
OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Catastrophe Event Ratio | 0.40%wtm_CatastropheEventRatio / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |  |  |  |  |
Reinsurance Recoverables | 173,800,000 us-gaap_ReinsuranceRecoverables / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  | 173,800,000 us-gaap_ReinsuranceRecoverables / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 173,800,000 us-gaap_ReinsuranceRecoverables / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |
Reinsurance recoverables, percentage of total | 100.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  | 100.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 100.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |
OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on paid losses | 12,200,000 us-gaap_ReinsuranceReceivablesPaidLossesRecoverable / dei_LegalEntityAxis = wtm_OnebeaconMember |  |  | 12,200,000 us-gaap_ReinsuranceReceivablesPaidLossesRecoverable / dei_LegalEntityAxis = wtm_OnebeaconMember | 12,200,000 us-gaap_ReinsuranceReceivablesPaidLossesRecoverable / dei_LegalEntityAxis = wtm_OnebeaconMember |  |  |  |  |  |
Reinsurance recoverable on unpaid losses | 161,600,000 us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims / dei_LegalEntityAxis = wtm_OnebeaconMember |  |  | 161,600,000 us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims / dei_LegalEntityAxis = wtm_OnebeaconMember | 161,600,000 us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims / dei_LegalEntityAxis = wtm_OnebeaconMember |  |  |  |  |  |
Property Catastrophe Program [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage |  | 100.00%us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyCatastropheProgramMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation | 110,000,000 wtm_ReinsuranceRetentionPolicyExcessRetentionAmountReinsuredNetOfParticipation / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyCatastropheProgramMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |  |  |  |  |
Property Catastrophe Program [Member] | Maximum | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 130,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyCatastropheProgramMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |  |  |  |  |
Property Catastrophe Program [Member] | Minimum | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained | 20,000,000 us-gaap_ReinsuranceRetentionPolicyAmountRetained / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyCatastropheProgramMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |  |  |  |  |
Property Catastrophe Reinsurance [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 30,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyCatastropheReinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage | 100.00%us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyCatastropheReinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |  |  |  |  |
Property Catastrophe Reinsurance [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained | 10,000,000 us-gaap_ReinsuranceRetentionPolicyAmountRetained / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyCatastropheReinsuranceMember |  |  |  |  |  |  |  |  |  |
Casualty Reinsurance Program | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained | 10,000,000 us-gaap_ReinsuranceRetentionPolicyAmountRetained / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CasualtyReinsuranceProgramMember |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 60,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CasualtyReinsuranceProgramMember |  |  |  |  |  |  |  |  |  |
Property Per-Risk, Acts of Terrorism | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage | 100.00%us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyPerRiskActsofTerrorismMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |  |  |  |  |
Property Per-Risk, Acts of Terrorism | Maximum | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 100,000,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyPerRiskActsofTerrorismMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |  |  |  |  |
Property Per-Risk, Acts of Terrorism | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 100,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyPerRiskActsofTerrorismMember |  |  |  |  |  |  |  |  |  |
Property Per-Risk, Acts of Terrorism | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 5,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyPerRiskActsofTerrorismMember |  |  |  |  |  |  |  |  |  |
Property Per Risk Reinsurance Program | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 100,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyPerRiskReinsuranceProgramMember |  |  |  |  |  |  |  |  |  |
Property Per Risk Reinsurance Program | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 5,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyPerRiskReinsuranceProgramMember |  | 10,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyPerRiskReinsuranceProgramMember |  |  |  |  |  |  |  |
Individual Risk Facultative [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 100,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_IndividualRiskFacultativeMember |  |  |  |  |  |  |  |  |  |
Inland and Ocean Marine | Marine Treaty Per Occurrence | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained | 2,000,000 us-gaap_ReinsuranceRetentionPolicyAmountRetained / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_MarineTreatyPerOccurrenceMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_InlandandOceanMarineMember |  |  |  |  |  |  |  |  |  |
Inland and Ocean Marine | Marine Treaty | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 60,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_MarineTreatyMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_InlandandOceanMarineMember |  |  |  |  |  |  |  |  |  |
Inland and Ocean Marine | Layer One | Marine Treaty | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Property Catastrophe Reinsurance Treaty First Layer Co-Partipation Percentage | 60.00%wtm_PropertyCatastropheReinsuranceTreatyFirstLayerCoPartipationPercentage / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_MarineTreatyMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_InlandandOceanMarineMember / wtm_ReinsuranceTypeandTierIdentifierAxis = wtm_LayerOneMember |  |  |  |  |  |  |  |  |  |
Inland and Ocean Marine | Layer One | Marine Treaty | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 7,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_MarineTreatyMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_InlandandOceanMarineMember / wtm_ReinsuranceTypeandTierIdentifierAxis = wtm_LayerOneMember |  |  |  |  |  |  |  |  |  |
Inland and Ocean Marine | Layer One | Marine Treaty | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 2,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_MarineTreatyMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_InlandandOceanMarineMember / wtm_ReinsuranceTypeandTierIdentifierAxis = wtm_LayerOneMember |  |  |  |  |  |  |  |  |  |
Inland and Ocean Marine | Layer One | Property Per Risk Treaty | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained | 20,000,000 us-gaap_ReinsuranceRetentionPolicyAmountRetained / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyPerRiskTreatyMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_InlandandOceanMarineMember / wtm_ReinsuranceTypeandTierIdentifierAxis = wtm_LayerOneMember |  |  |  |  |  |  |  |  |  |
Inland and Ocean Marine | Layer One | Catastrophic Losses [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained | 2,000,000 us-gaap_ReinsuranceRetentionPolicyAmountRetained / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CatastrophicLossesMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_InlandandOceanMarineMember / wtm_ReinsuranceTypeandTierIdentifierAxis = wtm_LayerOneMember |  |  |  |  |  |  |  |  |  |
Annual Aggregate Deductible to Reinsurance Retention Policy | 1,500,000 wtm_AnnualAggregateDeductibletoReinsuranceRetentionPolicy / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CatastrophicLossesMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_InlandandOceanMarineMember / wtm_ReinsuranceTypeandTierIdentifierAxis = wtm_LayerOneMember |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 5,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CatastrophicLossesMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_InlandandOceanMarineMember / wtm_ReinsuranceTypeandTierIdentifierAxis = wtm_LayerOneMember |  |  |  |  |  |  |  |  |  |
Catastrophic Losses [Member] | Layer One | Catastrophic Losses [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Annual Aggregate Deductible to Reinsurance Retention Policy | 5,000,000 wtm_AnnualAggregateDeductibletoReinsuranceRetentionPolicy / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CatastrophicLossesMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CatastrophicLossesMember / wtm_ReinsuranceTypeandTierIdentifierAxis = wtm_LayerOneMember |  |  |  |  |  |  |  |  |  |
Health Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 10,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CasualtyReinsuranceProgramMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_HealthInsuranceProductLineMember |  |  |  |  |  |  |  |  |  |
Health Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 5,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CasualtyReinsuranceProgramMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_HealthInsuranceProductLineMember |  |  |  |  |  |  |  |  |  |
Health Insurance Product Line | HMO Provider Excess Reinsurance [Member] | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 5,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_HMOProviderExcessReinsuranceMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_HealthInsuranceProductLineMember |  |  |  |  |  |  |  |  |  |
Health Insurance Product Line | Layer One | Casualty Reinsurance Program | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage | 10.00%us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CasualtyReinsuranceProgramMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_HealthInsuranceProductLineMember / wtm_ReinsuranceTypeandTierIdentifierAxis = wtm_LayerOneMember |  |  |  |  |  |  |  |  |  |
Property, Liability and Casualty Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 11,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CasualtyReinsuranceProgramMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember |  |  |  |  |  |  |  |  |  |
Property, Liability and Casualty Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 5,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CasualtyReinsuranceProgramMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember |  |  |  |  |  |  |  |  |  |
Property, Liability and Casualty Insurance Product Line | Layer One | Casualty Reinsurance Program | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage | 10.00%us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CasualtyReinsuranceProgramMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember / wtm_ReinsuranceTypeandTierIdentifierAxis = wtm_LayerOneMember |  |  |  |  |  |  |  |  |  |
Workers compensation | Casualty Reinsurance Program | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage | 100.00%us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CasualtyReinsuranceProgramMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_WorkersCompensationMember |  |  |  |  |  |  |  |  |  |
Workers compensation | Casualty Reinsurance Program | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 10,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CasualtyReinsuranceProgramMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_WorkersCompensationMember |  |  |  |  |  |  |  |  |  |
Workers compensation | Casualty Reinsurance Program | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 2,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_CasualtyReinsuranceProgramMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_WorkersCompensationMember |  |  |  |  |  |  |  |  |  |
Healthcare Professional Liability [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 20,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_SecondCasualtyExcessofLossMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_HealthcareProfessionalLiabilityMember |  |  |  |  |  |  |  |  |  |
Healthcare Professional Liability [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 10,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_SecondCasualtyExcessofLossMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_HealthcareProfessionalLiabilityMember |  |  |  |  |  |  |  |  |  |
Other Casualty Business [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 21,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_SecondCasualtyExcessofLossMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_OtherCasualtyBusinessMember |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage | 100.00%us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_SecondCasualtyExcessofLossMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_OtherCasualtyBusinessMember |  |  |  |  |  |  |  |  |  |
Other Casualty Business [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 11,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_SecondCasualtyExcessofLossMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_OtherCasualtyBusinessMember |  |  |  |  |  |  |  |  |  |
Surety Product Line [Member] | Surety Reinsurance Program [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 40,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_SuretyReinsuranceProgramMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_SuretySegmentMember |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage |  |  |  | 100.00%us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_SuretyReinsuranceProgramMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_SuretySegmentMember |  |  |  |  |  |  |
Surety Product Line [Member] | Surety Reinsurance Program [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 80,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_SuretyReinsuranceProgramMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_SuretySegmentMember |  |  |  |  |  |  |
Surety Product Line [Member] | Surety Reinsurance Program [Member] | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 5,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_SuretyReinsuranceProgramMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_SuretySegmentMember |  |  |  |  |  |  |
Crop | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage | 48.50%us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CropinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage | 101.50%wtm_ReinsuranceRetentionPolicyLossRatioReinsuredriskPercentage / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CropinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |  |  |  |  |
Crop-hail insurance [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage | 80.00%us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CrophailinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage | 100.00%wtm_ReinsuranceRetentionPolicyLossRatioReinsuredriskPercentage / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CrophailinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |  |  |  |  |
Film Completion Bond Business [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  | 30,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_FilmCompletionBondBusinessMember |  |  |  |  |  |
Film Completion Bond Business [Member] | OneBeacon | Minimum |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  | 2,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_FilmCompletionBondBusinessMember |  |  |  |  |  |
Film Completion Bond Business [Member] | Facultative Treaty Layer [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  | 50,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_FacultativeTreatyLayerMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_FilmCompletionBondBusinessMember |  |  |  |  |  |
AA | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Recoverables | 53,800,000 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  | 53,800,000 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 53,800,000 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |
Reinsurance recoverables, percentage of total | 31.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  | 31.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 31.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |
A | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Recoverables | 93,200,000 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  | 93,200,000 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 93,200,000 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |
Reinsurance recoverables, percentage of total | 54.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  | 54.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 54.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |
BBB plus | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Recoverables | 26,800,000 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  | 26,800,000 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 26,800,000 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |
Reinsurance recoverables, percentage of total | 15.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  | 15.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 15.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |
OneBeacon Runoff | BBB plus | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Recoverables | 23,800,000 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  | 23,800,000 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 23,800,000 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |  |  |
Scenario, Forecast [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of certified losses required to be shared by federal government subject to terrorism risk insurance act |  |  |  |  |  | 80.00%wtm_PercentageOfCertifiedLossesRequiredToBeSharedByFederalGovernmentSubjectToTerrorismRiskInsuranceAct / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  | 85.00%wtm_PercentageOfCertifiedLossesRequiredToBeSharedByFederalGovernmentSubjectToTerrorismRiskInsuranceAct / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |  |
Decreasing Annual Percentage of Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act |  |  |  |  |  |  |  |  | 1.00%wtm_DecreasingAnnualPercentageofCertifiedLossesRequiredtobeSharedbyFederalGovernmentSubjecttoTerrorismRiskInsuranceAct / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |
Retained Percentage Relating To Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act |  |  |  |  |  | 20.00%wtm_RetainedPercentageRelatingToCertifiedLossesRequiredtobeSharedbyFederalGovernmentSubjecttoTerrorismRiskInsuranceAct / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  | 15.00%wtm_RetainedPercentageRelatingToCertifiedLossesRequiredtobeSharedbyFederalGovernmentSubjecttoTerrorismRiskInsuranceAct / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |  |
Increased Annual Retained Percentage Relating to Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act |  |  |  |  |  |  | 1.00%wtm_IncreasedAnnualRetainedPercentageRelatingtoCertifiedLossesRequiredtobeSharedbyFederalGovernmentSubjecttoTerrorismRiskInsuranceAct / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |  |  |
Scenario, Forecast [Member] | Property Per-Risk, Acts of Terrorism | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  |  |  |  |  | 120,000,000 us-gaap_ReinsuranceRetentionPolicyAmountRetained / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyPerRiskActsofTerrorismMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage |  |  |  |  |  |  |  | 85.00%us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyPerRiskActsofTerrorismMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |  |
Scenario, Forecast [Member] | Property Per-Risk, Acts of Terrorism | Maximum | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  |  |  |  |  | 100,000,000,000 us-gaap_ReinsuranceRetentionPolicyAmountRetained / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ReinsurancePolicyTypeAxis = wtm_PropertyPerRiskActsofTerrorismMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |  |
Department of Treasury [Member] | Scenario, Forecast [Member] | Property and Casualty, Commercial Insurance Product Line [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  | 200,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / us-gaap_RegulatoryAgencyAxis = wtm_DepartmentofTreasuryMember / us-gaap_ReinsurancePolicyTypeAxis = us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  | 100,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / us-gaap_RegulatoryAgencyAxis = wtm_DepartmentofTreasuryMember / us-gaap_ReinsurancePolicyTypeAxis = us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember | 20,000,000 us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured / us-gaap_RegulatoryAgencyAxis = wtm_DepartmentofTreasuryMember / us-gaap_ReinsurancePolicyTypeAxis = us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |

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- Definition Estimated amount after valuation allowance of reinsurance recoverables, due from reinsurers for incurred claims not yet reported, and claims settlement expenses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 + Details
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- Definition Amount after valuation allowance currently billed or billable under reinsurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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- Definition Amount after valuation allowance recoverable including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387 Reference 3: 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.(a),6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Percentage of risk reinsured on insurance products per entity policy. + References No definition available. + Details
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