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Fair Value of Financial Instruments (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |
Senior notes | $ 746.6 us-gaap_LongTermDebt | $ 676.4 us-gaap_LongTermDebt
Fair value | 2012 OBH Senior Notes |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |
Senior notes | 286.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = wtm_A2012ObhSeniorNotesMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 269.8 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = wtm_A2012ObhSeniorNotesMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Fair value | SIG Senior Notes |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |
Senior notes | 437.8 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = wtm_SiriusInternationalSeniorNotesMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 438.1 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = wtm_SiriusInternationalSeniorNotesMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Fair value | SIG Preference Shares |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |
Senior notes | 260.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = wtm_SIGPreferenceSharesMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 260.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = wtm_SIGPreferenceSharesMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Carrying Value | 2012 OBH Senior Notes |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |
Senior notes | 274.7 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = wtm_A2012ObhSeniorNotesMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 274.7 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = wtm_A2012ObhSeniorNotesMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Carrying Value | SIG Senior Notes |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |
Senior notes | 399.7 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = wtm_SiriusInternationalSeniorNotesMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 399.6 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = wtm_SiriusInternationalSeniorNotesMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Carrying Value | SIG Preference Shares |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions |  |
Senior notes | $ 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = wtm_SIGPreferenceSharesMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | $ 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = wtm_SIGPreferenceSharesMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember

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- Definition Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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