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Investment Securities (Investment Gains (Losses) For Level 3) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2011 | Dec. 31, 2012
Investment securities |  |  |  |
Realized gains | $ 270.0 us-gaap_TradingSecuritiesRealizedGain | $ 221.4 us-gaap_TradingSecuritiesRealizedGain | $ 162.2 us-gaap_TradingSecuritiesRealizedGain |
Realized losses | 36.2 us-gaap_TradingSecuritiesRealizedLoss | 116.9 us-gaap_TradingSecuritiesRealizedLoss | 94.1 us-gaap_TradingSecuritiesRealizedLoss |
Level 3 Inputs |  |  |  |
Investment securities |  |  |  |
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments | 3.7 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 9.8 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 17.7 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Fixed maturity investments | Level 3 Inputs |  |  |  |
Investment securities |  |  |  |
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments | 1.2 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | (2.3)wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember |  | 7.7 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Common equity securities | Level 3 Inputs |  |  |  |
Investment securities |  |  |  |
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments | 5.9 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 0.9 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 3.0 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Convertible fixed maturity and preferred investments | Level 3 Inputs |  |  |  |
Investment securities |  |  |  |
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments | 0.7 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 0 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember |  | 0 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Other long-term investments | Level 3 Inputs |  |  |  |
Investment securities |  |  |  |
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments | $ (4.1)wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | $ 11.2 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember |  | $ 7.0 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember

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- Definition This item represents the gain realized during the period from the sale of trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph c -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 3: 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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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- Definition This item represents the loss realized during the period from the sale of trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph c -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 3: 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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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