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Derivatives (Realized and Unrealized Gains (Losses)) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | 12 Months Ended |
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |
Change in fair value of derivatives, included in other revenue | $ (72.4)us-gaap_DerivativeGainLossOnDerivativeNet | $ (402.0)us-gaap_DerivativeGainLossOnDerivativeNet | $ (339.0)us-gaap_DerivativeGainLossOnDerivativeNet |
Derivative, Fair Value, Net | 56.4 us-gaap_DerivativeFairValueOfDerivativeNet | 69.2 us-gaap_DerivativeFairValueOfDerivativeNet | 98.3 us-gaap_DerivativeFairValueOfDerivativeNet | 290.4 us-gaap_DerivativeFairValueOfDerivativeNet
Fixed income/interest rate |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |
Change in fair value of derivatives, included in other revenue | (33.7)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (108.7)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (149.5)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |
Derivative, Fair Value, Net | (1.7)us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (9.7)us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |  |
Foreign exchange |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |
Change in fair value of derivatives, included in other revenue | (1.3)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (96.7)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (102.3)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |
Derivative, Fair Value, Net | 44.1 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 58.0 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |  |
Equity |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |
Change in fair value of derivatives, included in other revenue | (37.4)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EquityContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (196.6)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EquityContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (87.2)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EquityContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |
Derivative, Fair Value, Net | 14.0 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EquityContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 20.9 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EquityContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |  |
Level 3 | Projected Surrender Assumption [Member] |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |
Change in fair value of derivatives, included in other revenue | $ (0.2)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = wtm_ProjectedSurrenderAssumptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | $ 7.2 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = wtm_ProjectedSurrenderAssumptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |  |

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- Definition Fair value of the assets less the liabilities of a derivative or group of derivatives. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13495-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 + Details
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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5618551-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959 + Details
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