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Investment Securities (Net Realized 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, Realized Gain (Loss) | $ 25.9 us-gaap_TradingSecuritiesRealizedGainLoss | $ 25.3 us-gaap_TradingSecuritiesRealizedGainLoss
Trading securities, tax on realized holding gain (loss) on investments | (6.5)wtm_TradingsecuritiesTaxonRealizedHoldingGainLossonInvestments | (4.6)wtm_TradingsecuritiesTaxonRealizedHoldingGainLossonInvestments
Trading securities realized holding gain (loss) on investments after tax | 19.4 wtm_TradingSecuritiesRealizedHoldingGainLossonInvestmentsafterTax | 20.7 wtm_TradingSecuritiesRealizedHoldingGainLossonInvestmentsafterTax
Foreign Currency Transaction Gain (Loss), Realized | 26.9 us-gaap_ForeignCurrencyTransactionGainLossRealized | (3.6)us-gaap_ForeignCurrencyTransactionGainLossRealized
Trading securities, realized foreign currency transaction gain (loss) t | (6.7)wtm_TradingSecuritiesTaxonRealizedForeignCurrencyTransactionGainLossTax | 0.9 wtm_TradingSecuritiesTaxonRealizedForeignCurrencyTransactionGainLossTax
Trading securities, realized foreign currency gain (loss) net of tax | 20.2 wtm_TradingSecuritiesRealizedForeignCurrencyGainLossNetofTax | (2.7)wtm_TradingSecuritiesRealizedForeignCurrencyGainLossNetofTax
Trading Securities, Realized Gain (Loss), Total | 52.8 wtm_TradingSecuritiesRealizedGainLossTotal | 21.7 wtm_TradingSecuritiesRealizedGainLossTotal
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (13.2)wtm_TaxonNetRealizedGainLossandForeignCurrencyGainLossonTradingSecurities | (3.7)wtm_TaxonNetRealizedGainLossandForeignCurrencyGainLossonTradingSecurities
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 39.6 wtm_NetRealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesNetofTax | 18.0 wtm_NetRealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesNetofTax
Fixed maturity investments. |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss) | 9.8 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 4.4 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Foreign Currency Transaction Gain (Loss), Realized | 21.2 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | (3.5)us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Trading Securities, Realized Gain (Loss), Total | 31.0 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 0.9 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Short-term investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss) | 0 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 0 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember
Foreign Currency Transaction Gain (Loss), Realized | 3.5 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 0 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember
Trading Securities, Realized Gain (Loss), Total | 3.5 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 0 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember
Common equity securities |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss) | 24.4 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 18.9 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Foreign Currency Transaction Gain (Loss), Realized | 1.1 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | (0.1)us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Trading Securities, Realized Gain (Loss), Total | 25.5 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 18.8 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Convertible fixed maturity investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss) | (4.2)us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 2.4 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Foreign Currency Transaction Gain (Loss), Realized | 0 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 0 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Trading Securities, Realized Gain (Loss), Total | (4.2)wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 2.4 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Other long-term investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss) | (4.1)us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | (0.3)us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Foreign Currency Transaction Gain (Loss), Realized | 1.1 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 0 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Trading Securities, Realized Gain (Loss), Total | (3.0)wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | (0.3)wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Forward Contracts |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss) | 0 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember | (0.1)us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember
Foreign Currency Transaction Gain (Loss), Realized | 0 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember | 0 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember
Trading Securities, Realized Gain (Loss), Total | $ 0 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember | $ (0.1)wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember

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- Definition Amount before tax of foreign currency transaction realized gain (loss) recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450189&loc=d3e30690-110894 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450222&loc=d3e30840-110895 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6450189&loc=d3e30700-110894 + Details
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- Definition This item represents the gain (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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