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v2.4.1.9

Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Jun. 30, 2014 | Dec. 31, 2011
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured | 15.00%wtm_PercentageOfParValueOfPolicyReinsured |  |  |  |  |  |  |  | 15.00%wtm_PercentageOfParValueOfPolicyReinsured |  |  |  |
Accumulated losses | $ 3,011.4 us-gaap_RetainedEarningsAccumulatedDeficit |  |  |  | $ 2,802.3 us-gaap_RetainedEarningsAccumulatedDeficit |  |  |  | $ 3,011.4 us-gaap_RetainedEarningsAccumulatedDeficit | $ 2,802.3 us-gaap_RetainedEarningsAccumulatedDeficit |  |  |
Noncontrolling interest in VIE | 543.0 us-gaap_MinorityInterest |  |  |  | 491.9 us-gaap_MinorityInterest |  |  |  | 543.0 us-gaap_MinorityInterest | 491.9 us-gaap_MinorityInterest |  |  |
Non-controlling interest in consolidated subsidiaries | (15.0)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (11.2)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | 4.6 us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (0.5)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | 0.3 us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (1.1)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (11.1)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (0.5)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (22.1)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (12.4)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (14.0)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest |  |
Payments to Acquire Other Investments |  |  |  |  |  |  |  |  | 137.0 us-gaap_PaymentsToAcquireOtherInvestments | 36.3 us-gaap_PaymentsToAcquireOtherInvestments | 96.7 us-gaap_PaymentsToAcquireOtherInvestments |  |
HG Global |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured | 15.00%wtm_PercentageOfParValueOfPolicyReinsured / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |  |  |  |  |  |  | 15.00%wtm_PercentageOfParValueOfPolicyReinsured / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |  |  |
Noncontrolling interest in VIE | 17.9 us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |  |  | 16.6 us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |  |  | 17.9 us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember | 16.6 us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |  |
Star & Shield Insurance Exchange [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes | 17.0 us-gaap_SurplusNotes / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |  |  |  |  |  | 17.0 us-gaap_SurplusNotes / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |  |
Total assets related to consolidated VIE's | 13.5 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |  |  |  |  |  | 13.5 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |  |
Total liabilities related to consolidated VIE's | 25.9 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |  |  |  |  |  | 25.9 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |  |
Accumulated losses | 12.4 us-gaap_RetainedEarningsAccumulatedDeficit / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |  |  |  |  |  | 12.4 us-gaap_RetainedEarningsAccumulatedDeficit / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |  |
Prospector |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Total assets related to consolidated VIE's | 135.8 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember |  |  |  | 249.2 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember |  |  |  | 135.8 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember | 249.2 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember |  |  |
Total liabilities related to consolidated VIE's | 39.8 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember |  |  |  | 90.6 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember |  |  |  | 39.8 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember | 90.6 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember |  |  |
Noncontrolling interest in VIE | 31.1 us-gaap_MinorityInterest / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember |  |  |  | 46.2 us-gaap_MinorityInterest / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember |  |  |  | 31.1 us-gaap_MinorityInterest / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember | 46.2 us-gaap_MinorityInterest / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember |  |  |
Net Income (Loss) Attributable to Parent |  |  |  |  |  |  |  |  | 5.6 us-gaap_NetIncomeLoss / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember | 14.5 us-gaap_NetIncomeLoss / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember | 9.3 us-gaap_NetIncomeLoss / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember |  |
Non-controlling interest in consolidated subsidiaries |  |  |  |  |  |  |  |  | 1.9 us-gaap_NetIncomeLossAttributableToNoncontrollingInterest / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember | 4.0 us-gaap_NetIncomeLossAttributableToNoncontrollingInterest / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember | 2.7 us-gaap_NetIncomeLossAttributableToNoncontrollingInterest / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember |  |
Captial at risk, investment in the fund | 64.9 us-gaap_EquityMethodInvestmentAggregateCost / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember |  |  |  |  |  |  |  | 64.9 us-gaap_EquityMethodInvestmentAggregateCost / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember |  |  |  |
Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage |  |  |  |  |  |  |  |  |  | 67.60%us-gaap_VariableInterestEntityOwnershipPercentage / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_ProspectorFundsMember |  |  |
Houston General Insurance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes | 4.0 us-gaap_SurplusNotes / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |  |  |  |  |  |  | 4.0 us-gaap_SurplusNotes / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |  |  |
Total assets related to consolidated VIE's | 2.4 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |  |  | 2.6 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |  |  | 2.4 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember | 2.6 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |  |
Total liabilities related to consolidated VIE's | 4.0 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |  |  | 4.2 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |  |  | 4.0 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember | 4.2 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |  |
Accumulated losses | 1.8 us-gaap_RetainedEarningsAccumulatedDeficit / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |  |  |  |  |  |  | 1.8 us-gaap_RetainedEarningsAccumulatedDeficit / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |  |  |
Accrued interest on surplus notes | 0.2 wtm_AccruedInterestOnSurplusNotes / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |  |  |  |  |  |  | 0.2 wtm_AccruedInterestOnSurplusNotes / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |  |  |
TaClaro [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Other Investments | 2.0 us-gaap_PaymentsToAcquireOtherInvestments / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TaClaroMember |  | 1.5 us-gaap_PaymentsToAcquireOtherInvestments / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TaClaroMember |  |  |  |  |  | 3.0 us-gaap_PaymentsToAcquireOtherInvestments / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TaClaroMember |  |  |  |
Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage |  |  |  |  |  |  |  |  | 49.00%us-gaap_VariableInterestEntityOwnershipPercentage / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TaClaroMember | 27.00%us-gaap_VariableInterestEntityOwnershipPercentage / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TaClaroMember |  | 39.00%us-gaap_VariableInterestEntityOwnershipPercentage / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TaClaroMember |
Notes Issued | 1.7 us-gaap_NotesIssued1 / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TaClaroMember |  |  |  |  |  |  |  |  |  |  |  |
Tranzutary Holdings LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Total assets related to consolidated VIE's | 29.6 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TranzutaryHoldingsLLCMember |  |  |  |  |  |  |  | 29.6 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TranzutaryHoldingsLLCMember |  |  |  |
Total liabilities related to consolidated VIE's | 4.1 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TranzutaryHoldingsLLCMember |  |  |  |  |  |  |  | 4.1 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TranzutaryHoldingsLLCMember |  |  |  |
Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage |  |  |  |  |  |  |  |  | 36.00%us-gaap_VariableInterestEntityOwnershipPercentage / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TranzutaryHoldingsLLCMember |  |  |  |
Entities Owned by Tuckerman Fund I [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number Of Manufacturing Companies From Whom Shares Received |  |  |  |  |  |  |  | 2 wtm_NumberOfManufacturingCompaniesFromWhomSharesReceived / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_EntitiesOwnedByTuckermanFundIMember |  |  |  |  |
Common Stock | BAM |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of notes |  |  |  |  |  |  |  |  |  | 503.0 us-gaap_ProceedsFromIssuanceOfLongTermDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_BamMember |  |  |
Common Stock | Tuckerman Fund I [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage |  |  |  |  |  |  |  |  |  |  |  |  | 94.00%us-gaap_VariableInterestEntityOwnershipPercentage / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TuckermanFundIMember
BAM |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Right to elect directors on board number |  |  |  |  |  |  |  |  |  | 2 wtm_RightToElectDirectorsOnBoardNumber / dei_LegalEntityAxis = wtm_BamMember |  |  |
HG Global |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured | 15.00%wtm_PercentageOfParValueOfPolicyReinsured / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |  |  |  |  |  | 15.00%wtm_PercentageOfParValueOfPolicyReinsured / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |  |
Surplus notes | 503.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |  | 503.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |  | 503.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_HgGlobalMember | 503.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |
Noncontrolling interest in VIE | 17.9 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |  | 16.6 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |  | 17.9 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_HgGlobalMember | 16.6 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |
First Loss Reinsurance Treaty [Member] | HG Global |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured | 15.00%wtm_PercentageOfParValueOfPolicyReinsured / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember / us-gaap_TransactionTypeAxis = wtm_FirstLossReinsuranceTreatyMember |  |  |  |  |  |  |  | 15.00%wtm_PercentageOfParValueOfPolicyReinsured / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember / us-gaap_TransactionTypeAxis = wtm_FirstLossReinsuranceTreatyMember |  |  |  |
First Loss Reinsurance Treaty [Member] | BAM |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of Premiums | 60.00%wtm_PercentageofPremiums / dei_LegalEntityAxis = wtm_BamMember / us-gaap_TransactionTypeAxis = wtm_FirstLossReinsuranceTreatyMember |  |  |  |  |  |  |  | 60.00%wtm_PercentageofPremiums / dei_LegalEntityAxis = wtm_BamMember / us-gaap_TransactionTypeAxis = wtm_FirstLossReinsuranceTreatyMember |  |  |  |
First Loss Reinsurance Treaty [Member] | HG Global |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Trust | 2 wtm_NumberOfTrust / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_TransactionTypeAxis = wtm_FirstLossReinsuranceTreatyMember |  |  |  |  |  |  |  | 2 wtm_NumberOfTrust / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_TransactionTypeAxis = wtm_FirstLossReinsuranceTreatyMember |  |  |  |
Amendment [Member] | Houston General Insurance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes | 4.0 us-gaap_SurplusNotes / us-gaap_TransactionTypeAxis = wtm_AmendmentMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |  |  |  |  |  |  | 4.0 us-gaap_SurplusNotes / us-gaap_TransactionTypeAxis = wtm_AmendmentMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_HoustonGeneralInsuranceExchangeMember |  |  |  |
Common Stock | TaClaro [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stock Issued | 0.3 us-gaap_StockIssued1 / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TaClaroMember |  |  |  |  |  |  |  |  |  |  |  |
Other Intangible Assets [Member] | Tranzutary Holdings LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Total assets related to consolidated VIE's | $ 28.9 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_FairValueByAssetClassAxis = us-gaap_OtherIntangibleAssetsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TranzutaryHoldingsLLCMember |  |  |  |  |  |  |  | $ 28.9 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_FairValueByAssetClassAxis = us-gaap_OtherIntangibleAssetsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = wtm_TranzutaryHoldingsLLCMember |  |  |  |

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- Definition Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which is directly or indirectly attributable to that ownership interest in subsidiary equity which is not attributable to the parent (that is, noncontrolling interest, previously referred to as minority interest). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.31) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 27 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 20 -Article 7 + Details
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