XML 106 R138.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Common Shareholders’ Equity and Non-controlling Interests (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended | 1 Months Ended | 61 Months Ended |
--- | --- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | May 31, 2007 quarter Period | Jun. 30, 2022 | May 24, 2007
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |  |
Common shares repurchased and retired during the period |  | 1,329,640 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares |  |  |  |
Value of common shares repurchased and retired during the period | $ 134.5 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue | $ 79.8 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue | $ 669.1 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue |  |  |
Value of each common share repurchased and retired during the period (in dollars per share) | $ 617.00 us-gaap_TreasuryStockAcquiredAverageCostPerShare | $ 564.00 us-gaap_TreasuryStockAcquiredAverageCostPerShare | $ 503.00 us-gaap_TreasuryStockAcquiredAverageCostPerShare |  |  |
Common Shares Issued [Abstract] |  |  |  |  |  |
Stock Issued During Period, Shares, New Issues | 28,405 us-gaap_StockIssuedDuringPeriodSharesNewIssues | 27,310 us-gaap_StockIssuedDuringPeriodSharesNewIssues | 44,054 us-gaap_StockIssuedDuringPeriodSharesNewIssues |  |  |
Shares issued to directors (in shares) | 1,395 wtm_StockIssuedDuringPeriodToDirectors | 1,590 wtm_StockIssuedDuringPeriodToDirectors |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Cash dividends declared and paid | 6.2 us-gaap_PaymentsOfDividendsCommonStock | 6.2 us-gaap_PaymentsOfDividendsCommonStock | 6.6 us-gaap_PaymentsOfDividendsCommonStock |  |  |
Dividends delared and paid (in dollars per share) | $ 1.00 us-gaap_CommonStockDividendsPerShareCashPaid | $ 1.00 us-gaap_CommonStockDividendsPerShareCashPaid | $ 1.00 us-gaap_CommonStockDividendsPerShareCashPaid |  |  |
Non-controlling interests | 543.0 us-gaap_MinorityInterest | 491.9 us-gaap_MinorityInterest |  |  |  |
Non-controlling Interests |  |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |  |
Value of common shares repurchased and retired during the period | 0 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 0 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember | 0 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember |  |  |
Total Non-Controlling Excluding Reciprocals [Member] |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Non-controlling interests | 168.6 us-gaap_MinorityInterest / us-gaap_StatementEquityComponentsAxis = wtm_TotalNonControllingExcludingReciprocalsMember | 65.8 us-gaap_MinorityInterest / us-gaap_StatementEquityComponentsAxis = wtm_TotalNonControllingExcludingReciprocalsMember |  |  |  |
Common Stock |  |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |  |
Value of common shares repurchased and retired during the period |  |  | 669.1 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Allocation of fair value of net assets acquired to non-controlling interests |  |  | (2.2)wtm_NoncontrollingInterestLossRecognizedInEquity / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember |  |  |
Restricted Stock |  |  |  |  |  |
Common Shares Issued [Abstract] |  |  |  |  |  |
Stock Issued During Period, Shares, New Issues | 23,440 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = us-gaap_RestrictedStockMember | 25,720 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = us-gaap_RestrictedStockMember |  |  |  |
Performance Shares |  |  |  |  |  |
Common Shares Issued [Abstract] |  |  |  |  |  |
Stock Issued During Period, Shares, New Issues | 3,570 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = us-gaap_PerformanceSharesMember |  |  |  |  |
Common Stock |  |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |  |
Common shares repurchased and retired during the period | 217,879 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 141,535 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 1,329,640 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |
Combination Three [Member] | Performance Shares |  |  |  |  |  |
Common Shares Issued [Abstract] |  |  |  |  |  |
Stock Issued During Period, Shares, New Issues |  |  | 9,577 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAxis = wtm_CombinationThreeMember / us-gaap_StatementClassOfStockAxis = us-gaap_PerformanceSharesMember |  |  |
Combination One [Member] | Restricted Stock |  |  |  |  |  |
Common Shares Issued [Abstract] |  |  |  |  |  |
Stock Issued During Period, Shares, New Issues |  |  | 32,160 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAxis = wtm_CombinationOneMember / us-gaap_StatementClassOfStockAxis = us-gaap_RestrictedStockMember |  |  |
Combination Two [Member] |  |  |  |  |  |
Common Shares Issued [Abstract] |  |  |  |  |  |
Shares issued to directors (in shares) |  |  | 700 wtm_StockIssuedDuringPeriodToDirectors / us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAxis = wtm_CombinationTwoMember |  |  |
Combination Four [Member] |  |  |  |  |  |
Common Shares Issued [Abstract] |  |  |  |  |  |
Shares issued to directors (in shares) |  |  | 1,617 wtm_StockIssuedDuringPeriodToDirectors / us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAxis = wtm_CombinationFourMember |  |  |
General Board Authorization [Member] |  |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |  |
Additional authorized repurchase of common shares | 338,092 wtm_StockRepurchaseProgramNumberOfAdditionalSharesAuthorizedToBeRepurchased / us-gaap_TitleOfIndividualAxis = wtm_GeneralBoardAuthorizationMember |  |  |  |  |
Common shares repurchased and retired during the period | 207,404 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares / us-gaap_TitleOfIndividualAxis = wtm_GeneralBoardAuthorizationMember | 140,000 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares / us-gaap_TitleOfIndividualAxis = wtm_GeneralBoardAuthorizationMember | 502,801 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares / us-gaap_TitleOfIndividualAxis = wtm_GeneralBoardAuthorizationMember |  |  |
Value of common shares repurchased and retired during the period | 128.2 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_TitleOfIndividualAxis = wtm_GeneralBoardAuthorizationMember | 78.9 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_TitleOfIndividualAxis = wtm_GeneralBoardAuthorizationMember | 256.0 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_TitleOfIndividualAxis = wtm_GeneralBoardAuthorizationMember |  |  |
Value of each common share repurchased and retired during the period (in dollars per share) | $ 618.00 us-gaap_TreasuryStockAcquiredAverageCostPerShare / us-gaap_TitleOfIndividualAxis = wtm_GeneralBoardAuthorizationMember | $ 564.00 us-gaap_TreasuryStockAcquiredAverageCostPerShare / us-gaap_TitleOfIndividualAxis = wtm_GeneralBoardAuthorizationMember | $ 508.00 us-gaap_TreasuryStockAcquiredAverageCostPerShare / us-gaap_TitleOfIndividualAxis = wtm_GeneralBoardAuthorizationMember |  |  |
Repurchased for Employee Benefit Plans [Member] |  |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |  |
Common shares repurchased and retired during the period | 10,475 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares / us-gaap_TypeOfArrangementAxis = wtm_RepurchasedforEmployeeBenefitPlansMember |  |  |  |  |
Repurchased for Employee Benefit Plans [Member] | Combination Three [Member] |  |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |  |
Common shares repurchased and retired during the period |  | 1,535 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares / us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAxis = wtm_CombinationThreeMember / us-gaap_TypeOfArrangementAxis = wtm_RepurchasedforEmployeeBenefitPlansMember | 10,010 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares / us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAxis = wtm_CombinationThreeMember / us-gaap_TypeOfArrangementAxis = wtm_RepurchasedforEmployeeBenefitPlansMember |  |  |
Tender Offer [Member] | Combination Two [Member] |  |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |  |
Value of common shares repurchased and retired during the period |  |  | 408.6 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAxis = wtm_CombinationTwoMember / us-gaap_TypeOfArrangementAxis = wtm_TenderOfferMember |  |  |
Value of each common share repurchased and retired during the period (in dollars per share) |  |  | $ 500.00 us-gaap_TreasuryStockAcquiredAverageCostPerShare / us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAxis = wtm_CombinationTwoMember / us-gaap_TypeOfArrangementAxis = wtm_TenderOfferMember |  |  |
Tender Offer [Member] | Combination Two [Member] | Common Stock |  |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |  |
Common shares repurchased and retired during the period |  |  | 816,829 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares / us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAxis = wtm_CombinationTwoMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember / us-gaap_TypeOfArrangementAxis = wtm_TenderOfferMember |  |  |
White Mountains |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Capital contributions |  | 594.5 us-gaap_PartnersCapitalAccountContributions / dei_LegalEntityAxis = wtm_WhiteMountainsMember | 594.5 us-gaap_PartnersCapitalAccountContributions / dei_LegalEntityAxis = wtm_WhiteMountainsMember |  |  |
Sirius Internation Group Ltd | Preferred stocks |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Non-cumulative perpetual preference shares issued |  |  |  | 250,000,000 us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredStockMember |  |
Preferred Stock, Liquidation Preference Per Share |  |  |  |  |  | $ 1,000 us-gaap_PreferredStockLiquidationPreference / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredStockMember
Proceeds from Preference Shares |  |  |  | 245.7 wtm_ProceedsFromPreferenceShares / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredStockMember |  |
Payments of Stock Issuance Costs |  |  |  | 4.3 us-gaap_PaymentsOfStockIssuanceCosts / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredStockMember |  |
Right to elect directors on board number |  |  |  | 2 wtm_RightToElectDirectorsOnBoardNumber / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredStockMember |  |
Number of Quarterly Period for Non Payment of Dividend which Give Right to Elect Directors on Board |  |  |  | 6 wtm_NumberofQuarterlyPeriodforNonPaymentofDividendwhichGiveRighttoElectDirectorsonBoard / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredStockMember |  |
Minimum Required Payment Period for Dividend to Cease Right to Elect Directors on Board |  |  |  | 4 wtm_MinimumRequiredPaymentPeriodForDividendToCeaseRightToElectDirectorsOnBoard / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredStockMember |  |
Preference shares floating rate of dividend amount in excess of libor |  |  |  | 3.20%wtm_PreferenceSharesFloatingRateOfDividendAmountInExcessOfLibor / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredStockMember |  |
Preferred Stock Redemption Basis Spread on Treasury Issue Rate |  |  |  | 45.00%wtm_PreferredStockRedemptionBasisSpreadOnTreasuryIssueRate / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredStockMember |  |
OneBeacon Ltd. |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners | 24.70%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember | 24.80%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |  |  |
Non-controlling interests | 258.7 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember | 273.7 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |  |  |
HG/BAM [Member] |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners | 100.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_BuildAmericaMutualMember | 100.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_BuildAmericaMutualMember |  |  |  |
Non-controlling interests | (121.9)us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_BuildAmericaMutualMember | (97.6)us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_BuildAmericaMutualMember |  |  |  |
Star & Shield Insurance Exchange [Member] |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners | 100.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember | 0.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |  |
Non-controlling interests | (12.4)us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember | 0 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |  |
HG Global |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners | 3.10%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_HgGlobalMember | 2.70%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |  |
Non-controlling interests | 17.9 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_HgGlobalMember | 16.6 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |  |
QuoteLab [Member] |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners | 40.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_QuoteLabMember | 0.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_QuoteLabMember |  |  |  |
Non-controlling interests | 22.6 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_QuoteLabMember | 0 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_QuoteLabMember |  |  |  |
Tranzact [Member] |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners | 36.80%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_TranzactMember | 0.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_TranzactMember |  |  |  |
Non-controlling interests | 88.2 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_TranzactMember | 0 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_TranzactMember |  |  |  |
WOBI [Member] |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners | 36.70%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_WOBIMember | 0.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_WOBIMember |  |  |  |
Non-controlling interests | 5.4 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_WOBIMember | 0 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_WOBIMember |  |  |  |
Prospector Offshore Fund Ltd [Member] |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners | 23.40%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_ProspectorOffshoreFundLtdMember | 28.30%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_ProspectorOffshoreFundLtdMember |  |  |  |
Non-controlling interests | 31.1 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_ProspectorOffshoreFundLtdMember | 32.1 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_ProspectorOffshoreFundLtdMember |  |  |  |
Prospector Turtle Fund [Member] |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners | 0.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_ProspectorTurtleFundMember | 31.10%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_ProspectorTurtleFundMember |  |  |  |
Non-controlling interests | 0 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_ProspectorTurtleFundMember | 14.1 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_ProspectorTurtleFundMember |  |  |  |
A W G Dewar Inc [Member] |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners | 18.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_WGDewarIncMember | 18.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_WGDewarIncMember |  |  |  |
Non-controlling interests | 3.4 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_WGDewarIncMember | 3.0 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_WGDewarIncMember |  |  |  |
SIG Preference Shares |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners | 100.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_SIGPreferenceSharesMember | 100.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_SIGPreferenceSharesMember |  |  |  |
Non-controlling interests | 250.0 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_SIGPreferenceSharesMember | 250.0 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_SIGPreferenceSharesMember |  |  |  |
BAM Management |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Capital contributions |  | 14.5 us-gaap_PartnersCapitalAccountContributions / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_BamManagementMember | 14.5 us-gaap_PartnersCapitalAccountContributions / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_BamManagementMember |  |  |
BAM Management | Non-controlling Interests |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Allocation of fair value of net assets acquired to non-controlling interests |  |  | 2.2 wtm_NoncontrollingInterestLossRecognizedInEquity / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_BamManagementMember / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember |  |  |
Interest Rate Cap | Sirius Internation Group Ltd |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
years of interest rate cap | 5 years | 5 years |  |  |  |
Derivative, Cap Interest Rate | 8.30%us-gaap_DerivativeCapInterestRate / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember | 8.30%us-gaap_DerivativeCapInterestRate / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember |  |  |  |
Interest Rate Cap | Sirius Internation Group Ltd | Preferred stocks |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Non-cumulative perpetual preference shares issued |  |  |  | 250,000,000 us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredStockMember |  |
Preference shares reference rate for dividend |  |  |  | 3 |  |
Preferred stock dividend rate |  |  |  | 7.506%us-gaap_PreferredStockDividendRatePercentage / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredStockMember |  |
OneBeacon |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
Non-controlling interests | $ 262.1 us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | $ 276.8 us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |
OneBeacon | Scenario, Forecast [Member] |  |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |  |
years of interest rate cap |  |  |  |  | 5 years |

X
- Definition Aggregate dividends paid during the period for each share of common stock outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
Name: | us-gaap_CommonStockDividendsPerShareCashPaid
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

X
- Definition Cap rate on an interest rate derivative such as an interest rate cap or collar. If market rates exceed the cap rate, a payment or receipt is triggered on the contract. + References No definition available. + Details
Name: | us-gaap_DerivativeCapInterestRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_DividendsCommonStockAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which is directly or indirectly attributable to that ownership interest in subsidiary equity which is not attributable to the parent (that is, noncontrolling interest, previously referred to as minority interest). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.31) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 27 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 20 -Article 7 + Details
Name: | us-gaap_MinorityInterest
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity. + References No definition available. + Details
Name: | us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Total contributions made by each class of partners (i.e., general, limited and preferred partners). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187171-122770 + Details
Name: | us-gaap_PartnersCapitalAccountContributions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of cash outflow in the form of ordinary dividends to common shareholders of the parent entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585 + Details
Name: | us-gaap_PaymentsOfDividendsCommonStock
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The cash outflow for cost incurred directly with the issuance of an equity security. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585 + Details
Name: | us-gaap_PaymentsOfStockIssuanceCosts
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The percentage rate used to calculate dividend payments on preferred stock. + References No definition available. + Details
Name: | us-gaap_PreferredStockDividendRatePercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The per share liquidation preference (or restrictions) of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) that has a preference in involuntary liquidation considerably in excess of the par or stated value of the shares. The liquidation preference is the difference between the preference in liquidation and the par or stated values of the share. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph d -Article 4 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(d)(1)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
Name: | us-gaap_PreferredStockLiquidationPreference
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition The number of shares issued or sold by the subsidiary or equity method investee per stock transaction. + References No definition available. + Details
Name: | us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of new stock issued during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5 + Details
Name: | us-gaap_StockIssuedDuringPeriodSharesNewIssues
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of shares that have been repurchased and retired during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5 + Details
Name: | us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Equity impact of the value of stock that has been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30, 31 -Article 5 + Details
Name: | us-gaap_StockRepurchasedAndRetiredDuringPeriodValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Total cost of shares repurchased divided by the total number of shares repurchased. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655 + Details
Name: | us-gaap_TreasuryStockAcquiredAverageCostPerShare
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | wtm_CommonSharesIssuedAbstract
Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | wtm_CommonSharesRepurchasedAndRetiredAbstract
Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the minimum number of quarters in which payment of dividends on the preference shares occurs, which will result in the cessation of the shareholders' right to elect directors. + References No definition available. + Details
Name: | wtm_MinimumRequiredPaymentPeriodForDividendToCeaseRightToElectDirectorsOnBoard
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
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 Represents the number of quarters whereby if no dividends are paid on the preference shares then the shareholders may elect two directors to the Sirius Group's board. + References No definition available. + Details
Name: | wtm_NumberofQuarterlyPeriodforNonPaymentofDividendwhichGiveRighttoElectDirectorsonBoard
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the amount in excess of LIBOR to be used in determining the floating annual rate of dividends on the preference shares. + References No definition available. + Details
Name: | wtm_PreferenceSharesFloatingRateOfDividendAmountInExcessOfLibor
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the reference rate used as a basis in determining the floating annual rate of dividend on the preference shares. + References No definition available. + Details
Name: | wtm_PreferenceSharesReferenceRateForDividend
Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the basis points (above a comparable treasury issue rate) that could be used to determine the present value for liquidating the preference shares if the Sirius Group elects to redeem the preference shares. + References No definition available. + Details
Name: | wtm_PreferredStockRedemptionBasisSpreadOnTreasuryIssueRate
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Proceeds from the issuance of preferred shares. + References No definition available. + Details
Name: | wtm_ProceedsFromPreferenceShares
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the number of directors who can be elected to the board by the holders of the preference shares, subject to certain specified conditions. + References No definition available. + Details
Name: | wtm_RightToElectDirectorsOnBoardNumber
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of shares issued during the period to directors. + References No definition available. + Details
Name: | wtm_StockIssuedDuringPeriodToDirectors
Namespace Prefix: | wtm_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of additional shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan. + References No definition available. + Details
Name: | wtm_StockRepurchaseProgramNumberOfAdditionalSharesAuthorizedToBeRepurchased
Namespace Prefix: | wtm_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Years of interest rate cap + References No definition available. + Details
Name: | wtm_Yearsofinterestratecap
Namespace Prefix: | wtm_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | dei_LegalEntityAxis=wtm_BuildAmericaMutualMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_HgGlobalMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_OneBeaconInsuranceGroupLtdMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_ProspectorOffshoreFundLtdMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_ProspectorTurtleFundMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_QuoteLabMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_SIGPreferenceSharesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_SiriusGroupLtdMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_StarShieldInsuranceExchangeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_TranzactMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_WGDewarIncMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_WhiteMountainsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_WOBIMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAxis=wtm_CombinationFourMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAxis=wtm_CombinationOneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAxis=wtm_CombinationThreeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAxis=wtm_CombinationTwoMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateCapMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=wtm_BamManagementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=wtm_OnebeaconMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=us-gaap_CommonStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=us-gaap_PerformanceSharesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=us-gaap_RestrictedStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=us-gaap_CommonStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=us-gaap_NoncontrollingInterestMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=wtm_TotalNonControllingExcludingReciprocalsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_TitleOfIndividualAxis=wtm_GeneralBoardAuthorizationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_TypeOfArrangementAxis=wtm_RepurchasedforEmployeeBenefitPlansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_TypeOfArrangementAxis=wtm_TenderOfferMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |