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Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2015 | Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Fixed maturity investments, at fair value | $ 4,771.9 us-gaap_TradingSecuritiesDebt | $ 4,784.3 us-gaap_TradingSecuritiesDebt
Common Stock |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 623.3 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 633.6 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Trading Securities, Unrealized Holding Gain | 163.4 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 175.1 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Trading Securities, Unrealized Holding Loss | (8.8)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | (5.2)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Net foreign currency gains (losses) | 9.0 wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | (1.9)wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Fixed maturity investments, at fair value | 786.9 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 801.6 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Convertible Debt Securities |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 13.1 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 19.1 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Trading Securities, Unrealized Holding Gain | 1.8 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 0.9 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Trading Securities, Unrealized Holding Loss | (0.3)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | (0.2)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Net foreign currency gains (losses) | 1.2 wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 0.7 wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Fixed maturity investments, at fair value | 15.8 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 20.5 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Other long-term investments. |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 339.8 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_OtherLongTermInvestmentsExcludingEquityMethodInvestmentsMember | 343.3 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_OtherLongTermInvestmentsExcludingEquityMethodInvestmentsMember
Trading Securities, Unrealized Holding Gain | 74.7 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_OtherLongTermInvestmentsExcludingEquityMethodInvestmentsMember | 73.0 us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_OtherLongTermInvestmentsExcludingEquityMethodInvestmentsMember
Trading Securities, Unrealized Holding Loss | (13.1)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_OtherLongTermInvestmentsExcludingEquityMethodInvestmentsMember | (10.9)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_OtherLongTermInvestmentsExcludingEquityMethodInvestmentsMember
Net foreign currency gains (losses) | 1.2 wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_OtherLongTermInvestmentsExcludingEquityMethodInvestmentsMember | 1.6 wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_OtherLongTermInvestmentsExcludingEquityMethodInvestmentsMember
Fixed maturity investments, at fair value | $ 402.6 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_OtherLongTermInvestmentsExcludingEquityMethodInvestmentsMember | $ 407.0 us-gaap_TradingSecuritiesDebt / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_OtherLongTermInvestmentsExcludingEquityMethodInvestmentsMember

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- Definition Cost of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading. + References No definition available. + Details
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- Definition Amount of investments in debt securities 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) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Amount of unrealized holding gain before loss recognized in the income statement for investments in trading securities. + 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 -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 Reference 3: 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 Amount of unrealized holding loss before gain recognized in the income statement for investments in trading securities. + 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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