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
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 $ 483,900,000us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims   $ 428,100,000us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims $ 483,900,000us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims $ 483,900,000us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims         $ 429,100,000us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims $ 2,507,300,000us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims
OneBeacon                      
Effects of Reinsurance [Line Items]                      
Catastrophe Event Ratio 0.40%wtm_CatastropheEventRatio
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_OnebeaconMember		                    
Reinsurance Recoverables 173,800,000us-gaap_ReinsuranceRecoverables
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_OnebeaconMember		     173,800,000us-gaap_ReinsuranceRecoverables
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_OnebeaconMember		 173,800,000us-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 12,200,000us-gaap_ReinsuranceReceivablesPaidLossesRecoverable
/ dei_LegalEntityAxis
= wtm_OnebeaconMember		     12,200,000us-gaap_ReinsuranceReceivablesPaidLossesRecoverable
/ dei_LegalEntityAxis
= wtm_OnebeaconMember		 12,200,000us-gaap_ReinsuranceReceivablesPaidLossesRecoverable
/ dei_LegalEntityAxis
= wtm_OnebeaconMember		            
Reinsurance recoverable on unpaid losses 161,600,000us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims
/ dei_LegalEntityAxis
= wtm_OnebeaconMember		     161,600,000us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims
/ dei_LegalEntityAxis
= wtm_OnebeaconMember		 161,600,000us-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 110,000,000wtm_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 130,000,000us-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 20,000,000us-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 30,000,000us-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 10,000,000us-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 10,000,000us-gaap_ReinsuranceRetentionPolicyAmountRetained
/ dei_LegalEntityAxis
= wtm_OnebeaconMember
/ us-gaap_ReinsurancePolicyTypeAxis
= wtm_CasualtyReinsuranceProgramMember		                    
Reinsurance retention policy, excess retention, amount reinsured 60,000,000us-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 100,000,000,000us-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 100,000,000us-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 5,000,000us-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 100,000,000us-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 5,000,000us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured
/ dei_LegalEntityAxis
= wtm_OnebeaconMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ReinsurancePolicyTypeAxis
= wtm_PropertyPerRiskReinsuranceProgramMember		   10,000,000us-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 100,000,000us-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 2,000,000us-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 60,000,000us-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 7,000,000us-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 2,000,000us-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 20,000,000us-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 2,000,000us-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 1,500,000wtm_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 5,000,000us-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 5,000,000wtm_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 10,000,000us-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 5,000,000us-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 5,000,000us-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 11,000,000us-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 5,000,000us-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 10,000,000us-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 2,000,000us-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 20,000,000us-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 10,000,000us-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 21,000,000us-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 11,000,000us-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       40,000,000us-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       80,000,000us-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       5,000,000us-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         30,000,000us-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         2,000,000us-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         50,000,000us-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 53,800,000us-gaap_ReinsuranceRecoverables
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAARatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_OnebeaconMember		     53,800,000us-gaap_ReinsuranceRecoverables
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAARatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_OnebeaconMember		 53,800,000us-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 93,200,000us-gaap_ReinsuranceRecoverables
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsARatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_OnebeaconMember		     93,200,000us-gaap_ReinsuranceRecoverables
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsARatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_OnebeaconMember		 93,200,000us-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 26,800,000us-gaap_ReinsuranceRecoverables
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBPlusRatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_OnebeaconMember		     26,800,000us-gaap_ReinsuranceRecoverables
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBPlusRatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_OnebeaconMember		 26,800,000us-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 23,800,000us-gaap_ReinsuranceRecoverables
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBPlusRatingMember
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= wtm_OnebeaconRunoffMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_OnebeaconMember		     23,800,000us-gaap_ReinsuranceRecoverables
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBPlusRatingMember
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= wtm_OnebeaconRunoffMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_OnebeaconMember		 23,800,000us-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               120,000,000us-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               100,000,000,000us-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           200,000,000us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured
/ us-gaap_RegulatoryAgencyAxis
= wtm_DepartmentofTreasuryMember
/ us-gaap_ReinsurancePolicyTypeAxis
= us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_OnebeaconMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		   100,000,000us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured
/ us-gaap_RegulatoryAgencyAxis
= wtm_DepartmentofTreasuryMember
/ us-gaap_ReinsurancePolicyTypeAxis
= us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_OnebeaconMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		 20,000,000us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured
/ us-gaap_RegulatoryAgencyAxis
= wtm_DepartmentofTreasuryMember
/ us-gaap_ReinsurancePolicyTypeAxis
= us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_OnebeaconMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember