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CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (USD $) In Millions, unless otherwise specified | Total | Common shares and paid-in surplus | Retained earnings | AOCI, after-tax | Parent [Member] | Total | Non-controlling Interests | Total Equity includes Noncontrolling Interest [Member]
--- | --- | --- | --- | --- | --- | --- | --- | ---
Beginning balance at Dec. 31, 2011 |  | $ 1,261.3 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | $ 2,789.7 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | $ 36.7 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | $ 4,087.7 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | $ 580.2 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | $ 4,667.9 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |
Net income | 193.4 us-gaap_ProfitLoss | 0 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 207.4 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | 207.4 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | (14.0)us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 193.4 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Net change in unrealized gains (losses) on investments in unconsolidated affiliates, after tax | 57.7 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates | 0 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 57.7 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | 57.7 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | 0 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 57.7 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Change in foreign currency translation, net of tax | 39.6 us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax | 0 us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 39.6 us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | 39.6 us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | 0 us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 39.6 us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Net change in pension liability and other accumulated comprehensive items | (2.9)wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease | 0 wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | (2.1)wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | (2.1)wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | (0.8)wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | (2.9)wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Comprehensive income | 301.8 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest | 0 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 207.4 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 95.2 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | 302.6 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | (14.8)us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 287.8 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Dividends declared on common shares |  | 0 us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | (6.6)us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | (6.6)us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | 0 us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | (6.6)us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Dividends/distributions to non-controlling interests |  | 0 us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | 0 us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | (39.1)us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | (39.1)us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Issuances of common shares |  | 5.8 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | 5.8 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | 0 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 5.8 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Repurchases and retirements of common shares | (669.1)us-gaap_StockRepurchasedAndRetiredDuringPeriodValue | (221.3)us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | (447.8)us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | (669.1)us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | 0 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | (669.1)us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Contribution or Distribution to/ from Non-Controlling Interests |  | 0 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | 0 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | 1.6 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 1.6 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Amortization of restricted share and option awards |  | 13.6 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | 13.6 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | 0.8 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 14.4 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Allocation of fair value of net assets acquired to non-controlling interests |  | (2.2)wtm_NoncontrollingInterestLossRecognizedInEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 wtm_NoncontrollingInterestLossRecognizedInEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 wtm_NoncontrollingInterestLossRecognizedInEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | (2.2)wtm_NoncontrollingInterestLossRecognizedInEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember |  | 0 wtm_NoncontrollingInterestLossRecognizedInEquity / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Deconsolidation of Hamer and Bri-Mar |  | 0 us-gaap_NoncontrollingInterestDecreaseFromDeconsolidation / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 us-gaap_NoncontrollingInterestDecreaseFromDeconsolidation / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_NoncontrollingInterestDecreaseFromDeconsolidation / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | 0 us-gaap_NoncontrollingInterestDecreaseFromDeconsolidation / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | (4.5)us-gaap_NoncontrollingInterestDecreaseFromDeconsolidation / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | (4.5)us-gaap_NoncontrollingInterestDecreaseFromDeconsolidation / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Ending balance at Dec. 31, 2012 |  | 1,057.2 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 2,542.7 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 131.9 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | 3,731.8 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | 526.4 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 4,258.2 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |
Net income | 309.4 us-gaap_ProfitLoss | 0 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 321.8 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 321.8 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | (12.4)us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 309.4 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Net change in unrealized gains (losses) on investments in unconsolidated affiliates, after tax | (98.1)wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates | 0 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | (98.1)wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | (98.1)wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | 0 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | (98.1)wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Accumulated Other Comprehensive Income (Loss) Foreign Currency Translation |  | 0 wtm_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslation / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 wtm_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslation / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 2.7 wtm_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslation / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 2.7 wtm_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslation / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | 0 wtm_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslation / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 2.7 wtm_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslation / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Change in foreign currency translation, net of tax | 2.7 us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax |  |  |  |  |  |  |
Net change in pension liability and other accumulated comprehensive items | 20.8 wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease | 0 wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 15.6 wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 15.6 wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | 5.2 wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 20.8 wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Comprehensive income | 247.2 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest | 0 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 321.8 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | (79.8)us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | 242.0 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | (7.2)us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 234.8 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Dividends declared on common shares |  | 0 us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | (6.2)us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | (6.2)us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | 0 us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | (6.2)us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Dividends/distributions to non-controlling interests |  | 0 us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 0 us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | (39.7)us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | (39.7)us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Issuances of common shares |  | 1.0 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 1.0 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | 0 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 1.0 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Repurchases and retirements of common shares | (79.8)us-gaap_StockRepurchasedAndRetiredDuringPeriodValue | (23.8)us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | (56.0)us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | (79.8)us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | 0 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | (79.8)us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Contribution or Distribution to/ from Non-Controlling Interests |  | 0 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 0 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | 11.5 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 11.5 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Amortization of restricted share and option awards |  | 16.7 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 16.7 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | 0.9 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 17.6 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Ending balance at Dec. 31, 2013 | 3,905.5 us-gaap_StockholdersEquity | 1,051.1 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 2,802.3 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 52.1 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | 3,905.5 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | 491.9 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 4,397.4 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |
Net income | 290.6 us-gaap_ProfitLoss | 0 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 312.7 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 312.7 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | (22.1)us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 290.6 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Net change in unrealized gains (losses) on investments in unconsolidated affiliates, after tax | 75.3 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates | 0 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 75.3 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 75.3 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | 0 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 75.3 wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Accumulated Other Comprehensive Income (Loss) Foreign Currency Translation |  | 0 wtm_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslation / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 wtm_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslation / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | (168.2)wtm_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslation / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | (168.2)wtm_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslation / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | (0.3)wtm_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslation / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | (168.5)wtm_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslation / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Change in foreign currency translation, net of tax | (168.5)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax |  |  |  |  |  |  |
Net change in pension liability and other accumulated comprehensive items | (11.7)wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease | 0 wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | (8.7)wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | (8.7)wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | (3.0)wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | (11.7)wtm_AccumulatedOtherComprehensiveIncomeLossNetOfTaxOtherPeriodIncreaseDecrease / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Comprehensive income | 207.8 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest | 0 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 312.7 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | (101.6)us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | 211.1 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | (25.4)us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 185.7 us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Dividends declared on common shares |  | 0 us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | (6.2)us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | (6.2)us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | 0 us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | (6.2)us-gaap_DividendsCommonStockCash / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Dividends/distributions to non-controlling interests |  | 0 us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 0 us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | (40.7)us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | (40.7)us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Issuances of common shares |  | 4.8 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 4.8 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | 0 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 4.8 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
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Contribution or Distribution to/ from Non-Controlling Interests |  | 0 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 0 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | 8.3 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 8.3 wtm_ChangeInNoncontrollingOwnership / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
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Partners' Capital Account, Redemptions |  | 0 us-gaap_PartnersCapitalAccountRedemptions / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 us-gaap_PartnersCapitalAccountRedemptions / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_PartnersCapitalAccountRedemptions / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 0 us-gaap_PartnersCapitalAccountRedemptions / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | (14.0)us-gaap_PartnersCapitalAccountRedemptions / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | (14.0)us-gaap_PartnersCapitalAccountRedemptions / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Amortization of restricted share and option awards |  | 15.9 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | 0 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | 0 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 15.9 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember |  | 0.8 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 16.7 us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember
Ending balance at Dec. 31, 2014 | $ 3,996.6 us-gaap_StockholdersEquity | $ 1,034.7 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember | $ 3,011.4 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember | $ (49.5)us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |  | $ 3,996.6 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | $ 543.0 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | $ 4,539.6 us-gaap_StockholdersEquity / us-gaap_StatementEquityComponentsAxis = wtm_TotalEquityIncludesNoncontrollingInterestMember

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- Definition Change in noncontrolling interest ownership, excluding net income to noncontrolling interests, dividends paid to noncontrolling interests, noncontrolling interest in comprehensive gains (losses), cumulative effect adjustments relating to changes in accounting principles related to noncontrolling interests and noncontrolling interest in the amortization of restricted share and option awards. + References No definition available. + Details
Name: | wtm_ChangeInNoncontrollingOwnership
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount disclosed represents the portion of the gain a subsidiary expects to recognize that would be for the benefit of the noncontrolling interest holders of that subsidiary, which was recognized as an adjustment between paid in surplus and noncontrolling interest in the prior year. + References No definition available. + Details
Name: | wtm_NoncontrollingInterestLossRecognizedInEquity
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Other Comprehensive Income (Loss) Equity in Unrealized Gains Losses from Investments in Unconsolidated Affiliates + References No definition available. + Details
Name: | wtm_OtherComprehensiveIncomeLossEquityinUnrealizedGainsLossesfromInvestmentsinUnconsolidatedAffiliates
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration