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Investments in Unconsolidated Affiliates (Symetra) (Details) (Symetra, USD $) In Millions, except Per Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended | 3 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 31, 2015 | Dec. 31, 2013 | Mar. 31, 2014 | Dec. 31, 2014 | Dec. 31, 2011 | Dec. 31, 2012
Investments in unconsolidated affiliates |  |  |  |  |  |
Shares of Symetra Financial Corporation owned (in shares) | 20.05 us-gaap_InvestmentsInAndAdvancesToAffiliatesBalanceShares |  |  | 20.05 us-gaap_InvestmentsInAndAdvancesToAffiliatesBalanceShares |  |
Ownership interest (as a percent) | 17.27%us-gaap_EquityMethodInvestmentOwnershipPercentage |  |  | 17.31%us-gaap_EquityMethodInvestmentOwnershipPercentage |  |
Equity Method Investments Fair Value Per Share |  |  |  |  | $ 15 wtm_EquityMethodInvestmentsFairValuePerShare |
Term of amortization (in years) |  | 30 years |  |  |  |
Weight Average Years Remaining | 28 years |  |  |  |  |
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity | $ 165.7 us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity |  |  | $ 170.4 us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity |  |
Common Stock |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |
Fair value of investment |  |  |  |  |  | 261.0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember
Share Price | $ 23.46 us-gaap_SharePrice / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |  |  |
equity in earnings of unconsolidated affiliates |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |
After tax amortization through equity in earnings | 0.7 wtm_InvestmentsInUnconsolidatedAffiliatesAmortizationThroughEquityInEarningsAfterTax / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_StatementEquityComponentsAxis = wtm_EquityinearningsofunconsolidatedaffiliatesMember |  | 0.7 wtm_InvestmentsInUnconsolidatedAffiliatesAmortizationThroughEquityInEarningsAfterTax / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_StatementEquityComponentsAxis = wtm_EquityinearningsofunconsolidatedaffiliatesMember |  |  |
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity | 35.1 us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_StatementEquityComponentsAxis = wtm_EquityinearningsofunconsolidatedaffiliatesMember |  |  | 36.4 us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_StatementEquityComponentsAxis = wtm_EquityinearningsofunconsolidatedaffiliatesMember |  |
Net unrealized gains (losses) of unconsolidated affiliates |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |
After tax amortization through equity in net unrealized losses | 2.8 wtm_InvestmentsInUnconsolidatedAffiliatesAmortizationThroughEquityInNetUnrealizedLossesAfterTax / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember |  | 2.9 wtm_InvestmentsInUnconsolidatedAffiliatesAmortizationThroughEquityInNetUnrealizedLossesAfterTax / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember |  |  |
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity | $ 130.6 us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember |  |  | $ 134.0 us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember |  |

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