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Investment Securities Investment Securities (Net Realized Investment Gain (losses) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Investment securities |  |  |
Trading Securities, Realized Gain (Loss) | $ 208.7 us-gaap_TradingSecuritiesRealizedGainLoss | $ 121.8 us-gaap_TradingSecuritiesRealizedGainLoss | $ 76.7 us-gaap_TradingSecuritiesRealizedGainLoss
Foreign Currency Transaction Gain (Loss), Realized | 25.1 us-gaap_ForeignCurrencyTransactionGainLossRealized | (17.3)us-gaap_ForeignCurrencyTransactionGainLossRealized | (8.6)us-gaap_ForeignCurrencyTransactionGainLossRealized
Net realized investment gains, pre-tax | 233.8 wtm_TradingSecuritiesRealizedGainLossTotal | 104.5 wtm_TradingSecuritiesRealizedGainLossTotal | 68.1 wtm_TradingSecuritiesRealizedGainLossTotal
Trading securities, Tax on Realized Holding Gain (Loss) on Investments | (39.7)wtm_TradingsecuritiesTaxonRealizedHoldingGainLossonInvestments | (30.0)wtm_TradingsecuritiesTaxonRealizedHoldingGainLossonInvestments | (23.8)wtm_TradingsecuritiesTaxonRealizedHoldingGainLossonInvestments
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax | (8.0)wtm_TradingSecuritiesTaxonRealizedForeignCurrencyTransactionGainLossTax | 5.5 wtm_TradingSecuritiesTaxonRealizedForeignCurrencyTransactionGainLossTax | 2.2 wtm_TradingSecuritiesTaxonRealizedForeignCurrencyTransactionGainLossTax
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (47.7)wtm_TaxonNetRealizedGainLossandForeignCurrencyGainLossonTradingSecurities | (24.5)wtm_TaxonNetRealizedGainLossandForeignCurrencyGainLossonTradingSecurities | (21.6)wtm_TaxonNetRealizedGainLossandForeignCurrencyGainLossonTradingSecurities
Trading Securities Realized Holding Gain (Loss) on Investments after Tax | 169.0 wtm_TradingSecuritiesRealizedHoldingGainLossonInvestmentsafterTax | 91.8 wtm_TradingSecuritiesRealizedHoldingGainLossonInvestmentsafterTax | 52.9 wtm_TradingSecuritiesRealizedHoldingGainLossonInvestmentsafterTax
Trading Securities, Realized Foreign Currency Gain (Loss) Net of Tax | 17.1 wtm_TradingSecuritiesRealizedForeignCurrencyGainLossNetofTax | (11.8)wtm_TradingSecuritiesRealizedForeignCurrencyGainLossNetofTax | (6.4)wtm_TradingSecuritiesRealizedForeignCurrencyGainLossNetofTax
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 186.1 wtm_NetRealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesNetofTax | 80.0 wtm_NetRealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesNetofTax | 46.5 wtm_NetRealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesNetofTax
Fixed maturity investments |  |  |
Investment securities |  |  |
Trading Securities, Realized Gain (Loss) | 23.9 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 8.6 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 82.5 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Foreign Currency Transaction Gain (Loss), Realized | 21.0 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | (15.0)us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | (2.5)us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Net realized investment gains, pre-tax | 44.9 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | (6.4)wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 80.0 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Short-term investments |  |  |
Investment securities |  |  |
Trading Securities, Realized Gain (Loss) | 0 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 0.1 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 | 2.9 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 0 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | (4.3)us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember
Net realized investment gains, pre-tax | 2.9 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 0.1 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | (4.3)wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember
Common equity securities |  |  |
Investment securities |  |  |
Trading Securities, Realized Gain (Loss) | 150.9 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 104.5 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 1.3 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Foreign Currency Transaction Gain (Loss), Realized | 0.9 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | (3.7)us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 0 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Net realized investment gains, pre-tax | 151.8 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 100.8 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 1.3 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Convertible fixed maturity and preferred investments |  |  |
Investment securities |  |  |
Trading Securities, Realized Gain (Loss) | 5.9 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 1.0 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | (8.2)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 | (1.8)us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Net realized investment gains, pre-tax | 5.9 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 1.0 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | (10.0)wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Other long-term investments |  |  |
Investment securities |  |  |
Trading Securities, Realized Gain (Loss) | 27.7 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 7.5 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 1.4 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Foreign Currency Transaction Gain (Loss), Realized | 0.3 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 1.4 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 0 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Net realized investment gains, pre-tax | 28.0 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 8.9 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 1.4 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Forward contracts |  |  |
Investment securities |  |  |
Trading Securities, Realized Gain (Loss) | 0.3 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember | 0.1 us-gaap_TradingSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember | (0.3)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 | 0 us-gaap_ForeignCurrencyTransactionGainLossRealized / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember
Net realized investment gains, pre-tax | $ 0.3 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember | $ 0.1 wtm_TradingSecuritiesRealizedGainLossTotal / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ForwardContractsMember | $ (0.3)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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