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Derivatives (Realized and Unrealized Gains (Losses) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014 | Dec. 31, 2013
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |
Realized and unrealized gains (losses) | $ (5.5)us-gaap_DerivativeGainLossOnDerivativeNet | $ (6.6)us-gaap_DerivativeGainLossOnDerivativeNet |  |
Derivative, fair value, net | 47.0 us-gaap_DerivativeFairValueOfDerivativeNet | 89.6 us-gaap_DerivativeFairValueOfDerivativeNet | 56.4 us-gaap_DerivativeFairValueOfDerivativeNet | 69.2 us-gaap_DerivativeFairValueOfDerivativeNet
Fixed income/interest rate |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |
Realized and unrealized gains (losses) | 9.3 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (6.4)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |  |
Derivative, fair value, net | 3.7 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |  | (1.7)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 |  |  |  |
Realized and unrealized gains (losses) | (12.1)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (6.5)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |  |
Derivative, fair value, net | 35.3 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |  | 44.1 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 |  |  |  |
Realized and unrealized gains (losses) | (2.7)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EquityContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 6.3 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EquityContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |  |
Derivative, fair value, net | $ 8.0 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EquityContractMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |  | $ 14.0 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EquityContractMember / 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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