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Investments in Unconsolidated Affiliates (Symetra Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended | 6 Months Ended
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Jun. 20, 2013
Investments in unconsolidated affiliates |  |  |  |  |
Common shares repurchased and retired during the period |  | 1,329,640 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares |  |  |
Treasury Stock Acquired, Average Cost Per Share | $ 617.00 us-gaap_TreasuryStockAcquiredAverageCostPerShare | $ 564.00 us-gaap_TreasuryStockAcquiredAverageCostPerShare | $ 503.00 us-gaap_TreasuryStockAcquiredAverageCostPerShare |  |
Symetra |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |
Shares of symetra financial corporation owned (in shares) | 20,050,000 us-gaap_InvestmentsInAndAdvancesToAffiliatesBalanceShares / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  |
Ownership interest (as a percent) | 17.31%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 17.03%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |
Dividends received | $ (34.1)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | $ (6.4)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | $ (4.9)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |
Investments in unconsolidated affiliates on conversion of warrants |  |  | 9,490,000 wtm_InvestmentsInUnconsolidatedAffiliatesOnConversionOfWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  | 2,650,000 wtm_InvestmentsInUnconsolidatedAffiliatesOnConversionOfWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember
Gain (Loss) on Warrants |  | 10.8 wtm_GainLossOnWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 17.7 wtm_GainLossOnWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |
Fair value of investment |  |  |  | 261.0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |
Estimated fair value of investment per share |  |  |  | $ 15 wtm_EquityMethodInvestmentsFairValuePerShare / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |
After-tax equity in losses of unconsolidated affiliates |  |  |  | 45.9 wtm_ImpairmentOfCommonSharesEquityInEarningsAfterTax / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |
Equity in net unrealized losses increase relating to amortization of common share impairment |  |  |  | 136.6 wtm_Othercomprehensiveincomeincreaserelatingtoamortizartionofcommonsharebasisdifference / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity | 2.4 us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 19.3 us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity, Attributable to Equity Earnings in Affiliates | 0.5 wtm_EquityMethodInvestmentDifferenceBetweenCarryingAmountandUnderlyingEquityAttributabletoEquityEarningsinAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  |
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity, Net Unrealized Gains and Losses of Affiliate | 1.9 wtm_EquityMethodInvestmentDifferenceBetweenCarryingAmountandUnderlyingEquityNetUnrealizedGainsandLossesofAffiliate / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  |
Term of amortization (in years) |  |  | 30 years |  |
Weight Average Years Remaining | 28 years |  |  |  |
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity, Unamortized Basis Difference Attributable to Equity Earnings | 36.4 wtm_EquityMethodInvestmentDifferenceBetweenCarryingAmountandUnderlyingEquityUnamortizedBasisDifferenceAttributabletoEquityEarnings / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  |
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity, Unamortized Basis Difference Attributable to Unrealized Gains in Affiliates | 134.0 wtm_EquityMethodInvestmentDifferenceBetweenCarryingAmountandUnderlyingEquityUnamortizedBasisDifferenceAttributabletoUnrealizedGainsinAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  |
Common Stock | Symetra |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |
Dividends received | (34.1)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | (6.4)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | (4.9)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |
Per share price, class of warrant (in dollars per share) | $ 23.05 us-gaap_SharePrice / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |  |
Gain (Loss) on Warrants |  | 0 wtm_GainLossOnWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 0 wtm_GainLossOnWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |
Warrants |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |
Gain (Loss) on Warrants |  |  |  |  | 10.8 wtm_GainLossOnWarrants / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember
Warrants | Symetra |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |
Dividends received | 0 us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 0 us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 0 us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember |  |
Per share price, class of warrant (in dollars per share) |  |  | $ 11.49 us-gaap_SharePrice / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember |  |
Gain (Loss) on Warrants |  | 10.8 wtm_GainLossOnWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 17.7 wtm_GainLossOnWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember |  |
Cash dividends received on warrants |  |  |  |  | 1.5 us-gaap_ProceedsFromDividendsReceived / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember
Symetra | Common Stock |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |
Common shares repurchased and retired during the period |  |  |  |  | 6,600,000 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / dei_LegalEntityAxis = wtm_SymetraMember
Treasury Stock Acquired, Average Cost Per Share |  |  |  |  | $ 13.44 us-gaap_TreasuryStockAcquiredAverageCostPerShare / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / dei_LegalEntityAxis = wtm_SymetraMember
equity in earnings of unconsolidated affiliates [Member] | Symetra |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity | 170.4 us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_StatementEquityComponentsAxis = wtm_EquityinearningsofunconsolidatedaffiliatesMember |  |  |  | 0.4 us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_StatementEquityComponentsAxis = wtm_EquityinearningsofunconsolidatedaffiliatesMember
After tax amortization through equity in earnings | 2.8 wtm_InvestmentsInUnconsolidatedAffiliatesAmortizationThroughEquityInEarningsAfterTax / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_StatementEquityComponentsAxis = wtm_EquityinearningsofunconsolidatedaffiliatesMember |  |  |  |
Net unrealized gains (losses) of unconsolidated affiliates [Member] | Symetra |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity |  |  |  |  | 18.9 us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember
After tax amortization through equity in net unrealized losses | $ 11.7 wtm_InvestmentsInUnconsolidatedAffiliatesAmortizationThroughEquityInNetUnrealizedLossesAfterTax / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember |  |  |  |

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