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Investments in Unconsolidated Affiliates (Financial Information) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended | 1 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Apr. 30, 2007 | Sep. 30, 2013 | Dec. 31, 2011
Symetra income statement data: [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax | $ 12.3 us-gaap_IncomeLossFromEquityMethodInvestments | $ 7.0 us-gaap_IncomeLossFromEquityMethodInvestments | $ 12.5 us-gaap_IncomeLossFromEquityMethodInvestments | $ 13.8 us-gaap_IncomeLossFromEquityMethodInvestments | $ 11.7 us-gaap_IncomeLossFromEquityMethodInvestments | $ 8.6 us-gaap_IncomeLossFromEquityMethodInvestments | $ 7.1 us-gaap_IncomeLossFromEquityMethodInvestments | $ 9.2 us-gaap_IncomeLossFromEquityMethodInvestments | $ 45.6 us-gaap_IncomeLossFromEquityMethodInvestments | $ 36.6 us-gaap_IncomeLossFromEquityMethodInvestments | $ 29.9 us-gaap_IncomeLossFromEquityMethodInvestments |  |  |
Investments in unconsolidated affiliates | 414.4 wtm_InvestmentsInUnconsolidatedAffiliates |  |  |  | 321.4 wtm_InvestmentsInUnconsolidatedAffiliates |  |  |  | 414.4 wtm_InvestmentsInUnconsolidatedAffiliates | 321.4 wtm_InvestmentsInUnconsolidatedAffiliates |  |  |  |
Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Symetra balance sheet data; [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total investments | 30,634.3 wtm_EquityMethodInvestmentSummarizedFinancialInformationInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 27,901.1 wtm_EquityMethodInvestmentSummarizedFinancialInformationInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 30,634.3 wtm_EquityMethodInvestmentSummarizedFinancialInformationInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 27,901.1 wtm_EquityMethodInvestmentSummarizedFinancialInformationInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  |
Separate account assets | 949.8 wtm_EquityMethodInvestmentSummarizedFinancialInformationSeparateAccountAssets / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 978.4 wtm_EquityMethodInvestmentSummarizedFinancialInformationSeparateAccountAssets / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 949.8 wtm_EquityMethodInvestmentSummarizedFinancialInformationSeparateAccountAssets / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 978.4 wtm_EquityMethodInvestmentSummarizedFinancialInformationSeparateAccountAssets / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  |
Total assets | 32,994.2 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAssets / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 30,129.5 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAssets / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 32,994.2 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAssets / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 30,129.5 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAssets / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  |
Policyholder liabilities | 27,276.0 wtm_EquityMethodInvestmentSummarizedFinancialInformationPolicyholderLiabilities / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 25,328.8 wtm_EquityMethodInvestmentSummarizedFinancialInformationPolicyholderLiabilities / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 27,276.0 wtm_EquityMethodInvestmentSummarizedFinancialInformationPolicyholderLiabilities / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 25,328.8 wtm_EquityMethodInvestmentSummarizedFinancialInformationPolicyholderLiabilities / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  |
Long-term debt | 697.2 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNoncurrentLiabilities / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 449.5 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNoncurrentLiabilities / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 697.2 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNoncurrentLiabilities / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 449.5 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNoncurrentLiabilities / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  |
Separate account liabilities | 949.8 wtm_EquityMethodInvestmentSummarizedFinancialInformationSeparateAccountsLiability / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 978.4 wtm_EquityMethodInvestmentSummarizedFinancialInformationSeparateAccountsLiability / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 949.8 wtm_EquityMethodInvestmentSummarizedFinancialInformationSeparateAccountsLiability / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 978.4 wtm_EquityMethodInvestmentSummarizedFinancialInformationSeparateAccountsLiability / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  |
Total liabilities | 29,633.6 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationLiabilities / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 27,187.6 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationLiabilities / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 29,633.6 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationLiabilities / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 27,187.6 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationLiabilities / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  |
Common shareholders’ equity | 3,360.6 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationEquityOrCapital / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 2,941.9 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationEquityOrCapital / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 3,360.6 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationEquityOrCapital / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 2,941.9 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationEquityOrCapital / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  |
Symetra income statement data: [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 629.1 wtm_EquityMethodInvestmentSummarizedFinancialInformationPremiumsEarnedNet / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 627.2 wtm_EquityMethodInvestmentSummarizedFinancialInformationPremiumsEarnedNet / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 605.0 wtm_EquityMethodInvestmentSummarizedFinancialInformationPremiumsEarnedNet / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |
Net investment income |  |  |  |  |  |  |  |  | 1,320.5 wtm_EquityMethodInvestmentSummarizedFinancialInformationNetInvestmentIncome / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 1,285.0 wtm_EquityMethodInvestmentSummarizedFinancialInformationNetInvestmentIncome / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 1,275.2 wtm_EquityMethodInvestmentSummarizedFinancialInformationNetInvestmentIncome / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |
Total revenues |  |  |  |  |  |  |  |  | 2,182.3 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 2,103.9 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 2,101.2 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |
Policy benefits |  |  |  |  |  |  |  |  | 1,399.6 wtm_EquityMethodInvestmentSummarizedFinancialInformationPolicyBenefits / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 1,394.9 wtm_EquityMethodInvestmentSummarizedFinancialInformationPolicyBenefits / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 1,371.8 wtm_EquityMethodInvestmentSummarizedFinancialInformationPolicyBenefits / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |
Total expenses |  |  |  |  |  |  |  |  | 1,882.4 wtm_EquityMethodInvestmentSummarizedFinancialInformationTotalExpenses / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 1,865.3 wtm_EquityMethodInvestmentSummarizedFinancialInformationTotalExpenses / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 1,831.1 wtm_EquityMethodInvestmentSummarizedFinancialInformationTotalExpenses / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |
Net income |  |  |  |  |  |  |  |  | 254.4 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 220.7 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 205.4 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |
Comprehensive net income (loss) |  |  |  |  |  |  |  |  | 397.0 wtm_EquityMethodInvestmentSummarizedFinancialInformationComprehensiveIncomeNetOfTax / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | (777.6)wtm_EquityMethodInvestmentSummarizedFinancialInformationComprehensiveIncomeNetOfTax / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 549.3 wtm_EquityMethodInvestmentSummarizedFinancialInformationComprehensiveIncomeNetOfTax / 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 |  |  |  | 17.31%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 17.03%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  | 47.0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 37.8 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 32.3 us-gaap_IncomeLossFromEquityMethodInvestments / 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 | 411.4 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 317.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 411.4 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 317.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 381.5 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  | 273.6 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember
Pentelia Capital Management |  |  |  |  |  |  |  |  |  |  |  |  |  |
Symetra income statement data: [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  |  |  |  | 33.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_PenteliaCapitalManagementMember |  |
Payments to acquire investment |  |  |  |  |  |  |  |  |  |  |  | 1.6 us-gaap_PaymentsToAcquireEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_PenteliaCapitalManagementMember |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  | 0.1 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_PenteliaCapitalManagementMember | (1.3)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_PenteliaCapitalManagementMember |  |  |  |
Dividends received |  |  |  |  |  |  |  |  | (0.4)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_PenteliaCapitalManagementMember |  |  |  |  |
Hamer, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |
Symetra income statement data: [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  | 1.9 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_HamerLlcMember | 0.9 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_HamerLlcMember | 0.4 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_HamerLlcMember |  |  |
Dividends received |  |  |  |  |  |  |  |  | (3.0)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_HamerLlcMember | (0.8)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_HamerLlcMember |  |  |  |
Investments in unconsolidated affiliates | 3.0 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_HamerLlcMember |  |  |  | 4.1 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_HamerLlcMember |  |  |  | 3.0 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_HamerLlcMember | 4.1 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_HamerLlcMember |  |  |  |
Bri-Mar Manufacturing, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |
Symetra income statement data: [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 0.9 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_BriMarManufacturingLlcMember |
Proceeds from Sale of Equity Method Investments |  |  |  |  |  |  |  |  |  | 1.1 us-gaap_ProceedsFromSaleOfEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_BriMarManufacturingLlcMember |  |  |  |
Equity Method Investment, Realized Gain (Loss) on Disposal |  |  |  |  |  |  |  |  |  | 1.7 us-gaap_EquityMethodInvestmentRealizedGainLossOnDisposal / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_BriMarManufacturingLlcMember |  |  |  |
Total | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Symetra income statement data: [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  | 47.0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 37.8 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 32.3 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |
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 |  |  |
Investments in unconsolidated affiliates | 411.4 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |  | 317.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |  | 411.4 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 317.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 351.2 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  | 261.0 wtm_InvestmentsInUnconsolidatedAffiliates / 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 |  |  |  |  |  |  |  | $ 23.05 us-gaap_SharePrice / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |  |  |
Total | Amortization of common share basis difference [Member] | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Symetra income statement data: [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  | $ 3.0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ExtraordinaryItemAxis = wtm_AmortizationofcommonsharebasisdifferenceMember / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | $ 3.0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ExtraordinaryItemAxis = wtm_AmortizationofcommonsharebasisdifferenceMember / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |  |

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- Definition This item represents disclosure of the amount of dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporation; these investments are accounted for under the equity method of accounting. This element excludes distributions that constitute a return of investment, which are classified as investing activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Article 5 + Details
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- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571 + Details
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- Definition Amount of gain (loss) on sale or disposal of an equity method investment. + References No definition available. + Details
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