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Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Change in Unrealized Holding Gain (Loss) | $ 9.5 us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss | $ 29.0 us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments | (2.8)wtm_TradingSecuritiesTaxonUnrealizedHoldingGainLossonInvestments | (9.1)wtm_TradingSecuritiesTaxonUnrealizedHoldingGainLossonInvestments
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax | 6.7 wtm_TradingSecuritiesUnrealizedHoldingGainLossonInvestmentsafterTax | 19.9 wtm_TradingSecuritiesUnrealizedHoldingGainLossonInvestmentsafterTax
Foreign Currency Transaction Gain (Loss), Unrealized | 41.1 us-gaap_ForeignCurrencyTransactionGainLossUnrealized | 13.1 us-gaap_ForeignCurrencyTransactionGainLossUnrealized
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax | (10.3)wtm_TradingSecuritiesUnrealizedForeignCurrencyTransactionGainLossTax | (2.9)wtm_TradingSecuritiesUnrealizedForeignCurrencyTransactionGainLossTax
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax | 30.8 wtm_TradingSecuritiesUnrealizedForeignCurrencyGainLossNetOfTax | 10.2 wtm_TradingSecuritiesUnrealizedForeignCurrencyGainLossNetOfTax
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 50.6 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities | 42.1 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax | (13.1)wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesTax | (12.0)wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesTax
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 37.5 wtm_NetUnrealizedGainLossAndForeignCurrencyGainLossOnTradingSecuritiesNetOfTax | 30.1 wtm_NetUnrealizedGainLossAndForeignCurrencyGainLossOnTradingSecuritiesNetOfTax
Fixed maturity investments. |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Change in Unrealized Holding Gain (Loss) | 12.1 us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 19.4 us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Foreign Currency Transaction Gain (Loss), Unrealized | 40.2 us-gaap_ForeignCurrencyTransactionGainLossUnrealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 12.2 us-gaap_ForeignCurrencyTransactionGainLossUnrealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 52.3 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 31.6 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Short-term investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Change in Unrealized Holding Gain (Loss) | 0 us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 0 us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember
Foreign Currency Transaction Gain (Loss), Unrealized | 0 us-gaap_ForeignCurrencyTransactionGainLossUnrealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 0 us-gaap_ForeignCurrencyTransactionGainLossUnrealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 0 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 0 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember
Common equity securities |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Change in Unrealized Holding Gain (Loss) | (4.2)us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 4.0 us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Foreign Currency Transaction Gain (Loss), Unrealized | 0.8 us-gaap_ForeignCurrencyTransactionGainLossUnrealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 0.5 us-gaap_ForeignCurrencyTransactionGainLossUnrealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (3.4)wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 4.5 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Convertible fixed maturity investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Change in Unrealized Holding Gain (Loss) | 0.7 us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 0.7 us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Foreign Currency Transaction Gain (Loss), Unrealized | 0.1 us-gaap_ForeignCurrencyTransactionGainLossUnrealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 0 us-gaap_ForeignCurrencyTransactionGainLossUnrealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 0.8 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 0.7 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Other long-term investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Change in Unrealized Holding Gain (Loss) | 0.9 us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 4.9 us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Foreign Currency Transaction Gain (Loss), Unrealized | 0 us-gaap_ForeignCurrencyTransactionGainLossUnrealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 0.4 us-gaap_ForeignCurrencyTransactionGainLossUnrealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 0.9 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 5.3 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Forward Contracts |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Change in Unrealized Holding Gain (Loss) | 0 us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember | 0 us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember
Foreign Currency Transaction Gain (Loss), Unrealized | 0 us-gaap_ForeignCurrencyTransactionGainLossUnrealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember | 0 us-gaap_ForeignCurrencyTransactionGainLossUnrealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | $ 0 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember | $ 0 wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember

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- Definition Amount of unrealized holding gain (loss) recognized in the income statement for investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading. + References Reference 1: 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 2: 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 3: 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 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 + Details
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- Definition The after-tax aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period. + References No definition available. + Details
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