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Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) (Hedge funds, USD $) In Millions, unless otherwise specified | Mar. 31, 2015
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Schedule of Trading Securities and Other Trading Assets [Line Items] |
Distributions from Inactive Hedge Funds | $ 2.1 wtm_DistributionsFromInactiveHedgeFunds
Other long-term investments | 92.1 us-gaap_TradingSecuritiesOther
Redemption requests | 6.9 wtm_FairValueInvestmentsEntitiesthatCalculateNetAssetValueRedemptionRequest
30-59 days notice |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 36.3 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod30To59DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember
Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member] |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 43.0 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod60To89DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember
Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member] |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 4.7 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod90To119DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember
120+ days notice |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 8.1 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriodOver120DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember
Monthly |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 4.5 us-gaap_TradingSecuritiesOther / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyMonthlyMember
Monthly | 30-59 days notice |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 4.5 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod30To59DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyMonthlyMember
Monthly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member] |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 0 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod60To89DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyMonthlyMember
Monthly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member] |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 0 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod90To119DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyMonthlyMember
Monthly | 120+ days notice |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 0 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriodOver120DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyMonthlyMember
Quarterly |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 59.8 us-gaap_TradingSecuritiesOther / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyQuarterlyMember
Quarterly | 30-59 days notice |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 31.8 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod30To59DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyQuarterlyMember
Quarterly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member] |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 20.1 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod60To89DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyQuarterlyMember
Quarterly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member] |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 0 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod90To119DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyQuarterlyMember
Quarterly | 120+ days notice |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 7.9 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriodOver120DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyQuarterlyMember
Semi-annual |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 22.9 us-gaap_TradingSecuritiesOther / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencySemiAnnualMember
Semi-annual | 30-59 days notice |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 0 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod30To59DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencySemiAnnualMember
Semi-annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member] |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 22.9 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod60To89DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencySemiAnnualMember
Semi-annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member] |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 0 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod90To119DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencySemiAnnualMember
Semi-annual | 120+ days notice |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 0 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriodOver120DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencySemiAnnualMember
Annual |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 4.9 us-gaap_TradingSecuritiesOther / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAnnualMember
Annual | 30-59 days notice |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 0 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod30To59DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAnnualMember
Annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member] |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 0 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod60To89DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAnnualMember
Annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member] |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | 4.7 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod90To119DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAnnualMember
Annual | 120+ days notice |
Schedule of Trading Securities and Other Trading Assets [Line Items] |
Other long-term investments | $ 0.2 us-gaap_TradingSecuritiesOther / wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriodOver120DaysMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember / wtm_RedemptionFrequencyAxis = wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAnnualMember

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- Definition Amount of other investments classified as trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562 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 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 5: 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.(a),1(c),(d),(e),(f),(g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph c, d, e, f, g -Article 7 + Details
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