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SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 3) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended | 1 Months Ended |
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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 | Dec. 31, 2013 | Dec. 31, 2011
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT |  |  |  |  |  |  |  |  |  |  |  |  |
Net income attributable to White Mountains’s common shareholders | $ 70.0 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 51.5 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 95.6 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 95.6 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 117.9 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 57.2 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 26.3 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 120.4 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 312.7 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 321.8 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 207.4 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic |  |
Charges (credits) to reconcile net income to net cash (used for) provided from operations: |  |  |  |  |  |  |  |  |  |  |  |  |
Net realized and unrealized investment gains |  |  |  |  |  |  |  |  | (283.9)us-gaap_GainLossOnInvestments | (161.7)us-gaap_GainLossOnInvestments | (118.2)us-gaap_GainLossOnInvestments |  |
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest | 12.2 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax | (6.7)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax | (2.6)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax | 0.5 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax | 0.3 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax | (0.4)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax | (3.9)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax | (0.5)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax | 1.8 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax | 42.1 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax | 24.0 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax |  |
Net change in other assets and liabilities, net |  |  |  |  |  |  |  |  | 57.1 us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet | 2.6 us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet | (103.5)us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet |  |
Net Cash Provided by (Used in) Operating Activities, Continuing Operations |  |  |  |  |  |  |  |  | 206.9 us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations | (28.7)us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations | (29.6)us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations |  |
Net cash used for discontinued operations |  |  |  |  |  |  |  |  | (88.1)us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations | (72.3)us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations | (196.2)us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations |  |
Net cash provided from (used for) operations |  |  |  |  |  |  |  |  | 118.8 us-gaap_NetCashProvidedByUsedInOperatingActivities | (101.0)us-gaap_NetCashProvidedByUsedInOperatingActivities | (225.8)us-gaap_NetCashProvidedByUsedInOperatingActivities |  |
Cash flows from investing activities: |  |  |  |  |  |  |  |  |  |  |  |  |
Net decrease in short-term investments |  |  |  |  |  |  |  |  | (351.5)us-gaap_PaymentsForProceedsFromShortTermInvestments | 34.8 us-gaap_PaymentsForProceedsFromShortTermInvestments | 145.3 us-gaap_PaymentsForProceedsFromShortTermInvestments |  |
Purchases of investment securities |  |  |  |  |  |  |  |  | (5,602.1)us-gaap_PaymentsToAcquireInvestments | (4,635.5)us-gaap_PaymentsToAcquireInvestments |  |  |
Sales and maturities of investment securities |  |  |  |  |  |  |  |  | 6,387.5 us-gaap_ProceedsFromSaleAndMaturityOfTradingSecuritiesHeldforinvestment | 4,979.8 us-gaap_ProceedsFromSaleAndMaturityOfTradingSecuritiesHeldforinvestment |  |  |
Receipt of cash flow from discontinued operations |  |  |  |  |  |  |  |  | (88.1)us-gaap_PaymentsForProceedsFromOtherInvestingActivities | (72.3)us-gaap_PaymentsForProceedsFromOtherInvestingActivities | (196.2)us-gaap_PaymentsForProceedsFromOtherInvestingActivities |  |
Net Cash Provided by (Used in) Investing Activities, Continuing Operations |  |  |  |  |  |  |  |  | (40.2)us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations | 329.5 us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations | 633.4 us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations |  |
Net cash provided from investing activities — discontinued operations |  |  |  |  |  |  |  |  | 88.1 us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations | 72.3 us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations | 196.2 us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations |  |
Net cash provided from investing activities |  |  |  |  |  |  |  |  | 47.9 us-gaap_NetCashProvidedByUsedInInvestingActivities | 401.8 us-gaap_NetCashProvidedByUsedInInvestingActivities | 829.6 us-gaap_NetCashProvidedByUsedInInvestingActivities |  |
Cash flows from financing activities: |  |  |  |  |  |  |  |  |  |  |  |  |
Draw down of revolving line of credit |  |  |  |  |  |  |  |  | 0 us-gaap_ProceedsFromLinesOfCredit | 200.0 us-gaap_ProceedsFromLinesOfCredit | 150.0 us-gaap_ProceedsFromLinesOfCredit |  |
Repayment of revolving line of credit |  |  |  |  |  |  |  |  | (66.5)us-gaap_RepaymentsOfDebt | (275.0)us-gaap_RepaymentsOfDebt | (75.0)us-gaap_RepaymentsOfDebt |  |
Repurchases and retirement of common shares |  |  |  |  |  |  |  |  | (128.2)us-gaap_PaymentsForRepurchaseOfCommonStock | (79.8)us-gaap_PaymentsForRepurchaseOfCommonStock | (669.1)us-gaap_PaymentsForRepurchaseOfCommonStock |  |
Dividends paid on common shares |  |  |  |  |  |  |  |  | (6.2)us-gaap_PaymentsOfDividendsCommonStock | (6.2)us-gaap_PaymentsOfDividendsCommonStock | (6.6)us-gaap_PaymentsOfDividendsCommonStock |  |
Net cash used for financing activities |  |  |  |  |  |  |  |  | (129.6)us-gaap_NetCashProvidedByUsedInFinancingActivities | (186.5)us-gaap_NetCashProvidedByUsedInFinancingActivities | (648.2)us-gaap_NetCashProvidedByUsedInFinancingActivities |  |
Net change in cash during the period |  |  |  |  |  |  |  |  | 22.8 us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease | 114.1 us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease | (41.3)us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease |  |
Cash balance at beginning of year | 349.5 us-gaap_CashAndCashEquivalentsAtCarryingValue |  |  |  | 326.7 us-gaap_CashAndCashEquivalentsAtCarryingValue |  |  |  | 349.5 us-gaap_CashAndCashEquivalentsAtCarryingValue | 326.7 us-gaap_CashAndCashEquivalentsAtCarryingValue | 212.6 us-gaap_CashAndCashEquivalentsAtCarryingValue | 326.7 us-gaap_CashAndCashEquivalentsAtCarryingValue | 251.9 us-gaap_CashAndCashEquivalentsAtCarryingValue
Cash balance at end of year | 349.5 us-gaap_CashAndCashEquivalentsAtCarryingValue |  |  |  | 326.7 us-gaap_CashAndCashEquivalentsAtCarryingValue |  |  |  | 349.5 us-gaap_CashAndCashEquivalentsAtCarryingValue | 326.7 us-gaap_CashAndCashEquivalentsAtCarryingValue | 212.6 us-gaap_CashAndCashEquivalentsAtCarryingValue | 326.7 us-gaap_CashAndCashEquivalentsAtCarryingValue | 251.9 us-gaap_CashAndCashEquivalentsAtCarryingValue
Interest Paid |  |  |  |  |  |  |  |  | (42.0)us-gaap_InterestPaid | (42.6)us-gaap_InterestPaid | (44.6)us-gaap_InterestPaid |  |
Common shares repurchased and retired during the period |  |  |  |  |  |  |  |  |  | 1,329,640 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares |  |  |
OneBeacon Ltd. common shares repurchased and retired |  |  |  |  |  |  |  |  | (1.7)wtm_PaymentforSubsidiarySharesRetired | 0 wtm_PaymentforSubsidiarySharesRetired | 0 wtm_PaymentforSubsidiarySharesRetired |  |
Lone Tree Insurance Group Ltd |  |  |  |  |  |  |  |  |  |  |  |  |
Cash flows from financing activities: |  |  |  |  |  |  |  |  |  |  |  |  |
Cash dividends received on warrants |  |  |  |  |  |  |  |  | 29.7 us-gaap_ProceedsFromDividendsReceived / dei_LegalEntityAxis = wtm_LoneTreeInsuranceGroupLtdMember |  |  | 28.0 us-gaap_ProceedsFromDividendsReceived / dei_LegalEntityAxis = wtm_LoneTreeInsuranceGroupLtdMember |
Noncash distribution received |  |  |  |  |  |  |  |  |  |  |  | 27.9 wtm_NoncashorPartNoncashDistributionReceived / dei_LegalEntityAxis = wtm_LoneTreeInsuranceGroupLtdMember |
White Mountains Insurance Group Ltd. |  |  |  |  |  |  |  |  |  |  |  |  |
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT |  |  |  |  |  |  |  |  |  |  |  |  |
Net income attributable to White Mountains’s common shareholders |  |  |  |  |  |  |  |  | 312.7 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 321.8 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 207.4 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Charges (credits) to reconcile net income to net cash (used for) provided from operations: |  |  |  |  |  |  |  |  |  |  |  |  |
Net realized and unrealized investment gains |  |  |  |  |  |  |  |  | (0.2)us-gaap_GainLossOnInvestments / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (0.2)us-gaap_GainLossOnInvestments / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (11.0)us-gaap_GainLossOnInvestments / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Undistributed current earnings from subsidiaries |  |  |  |  |  |  |  |  | (329.9)wtm_UndistributedEarningFromSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (359.7)wtm_UndistributedEarningFromSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (219.3)wtm_UndistributedEarningFromSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest |  |  |  |  |  |  |  |  | (13.9)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Other non-cash reconciling items including restricted share and option amortization |  |  |  |  |  |  |  |  | 12.9 wtm_OtherNonCashReconcilingItemsIncludingShareBasedCompensationShareAmortization / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 15.6 wtm_OtherNonCashReconcilingItemsIncludingShareBasedCompensationShareAmortization / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 22.1 wtm_OtherNonCashReconcilingItemsIncludingShareBasedCompensationShareAmortization / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Net change in other assets and liabilities, net |  |  |  |  |  |  |  |  | (7.6)us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (2.9)us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (10.0)us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Net Cash Provided by (Used in) Operating Activities, Continuing Operations |  |  |  |  |  |  |  |  | (26.0)us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (25.4)us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (10.8)us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Net cash used for discontinued operations |  |  |  |  |  |  |  |  | 13.9 us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0 us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0 us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Net cash provided from (used for) operations |  |  |  |  |  |  |  |  | (12.1)us-gaap_NetCashProvidedByUsedInOperatingActivities / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (25.4)us-gaap_NetCashProvidedByUsedInOperatingActivities / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (10.8)us-gaap_NetCashProvidedByUsedInOperatingActivities / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Cash flows from investing activities: |  |  |  |  |  |  |  |  |  |  |  |  |
Net decrease in short-term investments |  |  |  |  |  |  |  |  | (29.6)us-gaap_PaymentsForProceedsFromShortTermInvestments / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 7.5 us-gaap_PaymentsForProceedsFromShortTermInvestments / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 63.3 us-gaap_PaymentsForProceedsFromShortTermInvestments / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Purchases of investment securities |  |  |  |  |  |  |  |  | (7.6)us-gaap_PaymentsToAcquireInvestments / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (26.2)us-gaap_PaymentsToAcquireInvestments / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (706.2)us-gaap_PaymentsToAcquireInvestments / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Sales and maturities of investment securities |  |  |  |  |  |  |  |  | 39.4 us-gaap_ProceedsFromSaleAndMaturityOfTradingSecuritiesHeldforinvestment / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 61.1 us-gaap_ProceedsFromSaleAndMaturityOfTradingSecuritiesHeldforinvestment / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 2,009.7 us-gaap_ProceedsFromSaleAndMaturityOfTradingSecuritiesHeldforinvestment / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Issuance of debt (to) from subsidiaries |  |  |  |  |  |  |  |  | 144.6 wtm_IssuanceOfDebtToFromSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 153.9 wtm_IssuanceOfDebtToFromSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (121.0)wtm_IssuanceOfDebtToFromSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Repayment of debt (to) from subsidiaries |  |  |  |  |  |  |  |  | (30.0)wtm_RepaymentOfDebtFromSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (10.3)wtm_RepaymentOfDebtFromSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 28.5 wtm_RepaymentOfDebtFromSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Receipt of cash flow from discontinued operations |  |  |  |  |  |  |  |  | 13.9 us-gaap_PaymentsForProceedsFromOtherInvestingActivities / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0 us-gaap_PaymentsForProceedsFromOtherInvestingActivities / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0 us-gaap_PaymentsForProceedsFromOtherInvestingActivities / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Payments to subsidiaries |  |  |  |  |  |  |  |  | 0 us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0 us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (663.0)us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Distributions from subsidiaries |  |  |  |  |  |  |  |  | 29.7 wtm_ProceedsFromSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0.1 wtm_ProceedsFromSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0 wtm_ProceedsFromSubsidiaries / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Net Cash Provided by (Used in) Investing Activities, Continuing Operations |  |  |  |  |  |  |  |  | 160.4 us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 186.1 us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 611.3 us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Net cash provided from investing activities — discontinued operations |  |  |  |  |  |  |  |  | (13.9)us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0 us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0 us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Net cash provided from investing activities |  |  |  |  |  |  |  |  | 146.5 us-gaap_NetCashProvidedByUsedInInvestingActivities / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 186.1 us-gaap_NetCashProvidedByUsedInInvestingActivities / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 611.3 us-gaap_NetCashProvidedByUsedInInvestingActivities / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Cash flows from financing activities: |  |  |  |  |  |  |  |  |  |  |  |  |
Draw down of revolving line of credit |  |  |  |  |  |  |  |  | 65.0 us-gaap_ProceedsFromLinesOfCredit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 200.0 us-gaap_ProceedsFromLinesOfCredit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 150.0 us-gaap_ProceedsFromLinesOfCredit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Repayment of revolving line of credit |  |  |  |  |  |  |  |  | (65.0)us-gaap_RepaymentsOfDebt / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (275.0)us-gaap_RepaymentsOfDebt / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (75.0)us-gaap_RepaymentsOfDebt / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Proceeds from issuances of common shares |  |  |  |  |  |  |  |  | 0 us-gaap_ProceedsFromIssuanceOfCommonStock / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0 us-gaap_ProceedsFromIssuanceOfCommonStock / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0 us-gaap_ProceedsFromIssuanceOfCommonStock / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Repurchases and retirement of common shares |  |  |  |  |  |  |  |  | (128.2)us-gaap_PaymentsForRepurchaseOfCommonStock / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (79.8)us-gaap_PaymentsForRepurchaseOfCommonStock / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (669.1)us-gaap_PaymentsForRepurchaseOfCommonStock / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Dividends paid on common shares |  |  |  |  |  |  |  |  | (6.2)us-gaap_PaymentsOfDividendsCommonStock / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (6.2)us-gaap_PaymentsOfDividendsCommonStock / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (6.6)us-gaap_PaymentsOfDividendsCommonStock / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Net Cash Provided by (Used in) Financing Activities, Continuing Operations |  |  |  |  |  |  |  |  | (134.4)us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (161.0)us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (600.7)us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Cash Provided by (Used in) Financing Activities, Discontinued Operations |  |  |  |  |  |  |  |  | 0 us-gaap_CashProvidedByUsedInFinancingActivitiesDiscontinuedOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0 us-gaap_CashProvidedByUsedInFinancingActivitiesDiscontinuedOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0 us-gaap_CashProvidedByUsedInFinancingActivitiesDiscontinuedOperations / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Net cash used for financing activities |  |  |  |  |  |  |  |  | (134.4)us-gaap_NetCashProvidedByUsedInFinancingActivities / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (161.0)us-gaap_NetCashProvidedByUsedInFinancingActivities / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (600.7)us-gaap_NetCashProvidedByUsedInFinancingActivities / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Net change in cash during the period |  |  |  |  |  |  |  |  | 0 us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (0.3)us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (0.2)us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Cash balance at beginning of year | 0.2 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |  |  | 0.2 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |  |  | 0.2 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0.2 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0.5 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0.2 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0.7 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Cash balance at end of year | 0.2 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |  |  | 0.2 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |  |  | 0.2 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0.2 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0.5 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0.2 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 0.7 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Interest Paid |  |  |  |  |  |  |  |  | (0.3)us-gaap_InterestPaid / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (2.0)us-gaap_InterestPaid / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (12.4)us-gaap_InterestPaid / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
OneBeacon Ltd. common shares repurchased and retired |  |  |  |  |  |  |  |  |  |  | (663.0)wtm_PaymentforSubsidiarySharesRetired / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |
Fixed maturity investments | Lone Tree Insurance Group Ltd |  |  |  |  |  |  |  |  |  |  |  |  |
Cash flows from financing activities: |  |  |  |  |  |  |  |  |  |  |  |  |
Cash dividends received on warrants |  |  |  |  |  |  |  |  |  |  |  | 27.9 us-gaap_ProceedsFromDividendsReceived / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / dei_LegalEntityAxis = wtm_LoneTreeInsuranceGroupLtdMember |
Cash | Lone Tree Insurance Group Ltd |  |  |  |  |  |  |  |  |  |  |  |  |
Cash flows from financing activities: |  |  |  |  |  |  |  |  |  |  |  |  |
Cash dividends received on warrants |  |  |  |  |  |  |  |  |  |  |  | 0.1 us-gaap_ProceedsFromDividendsReceived / us-gaap_InvestmentTypeAxis = us-gaap_CashMember / dei_LegalEntityAxis = wtm_LoneTreeInsuranceGroupLtdMember |
Revolving Credit Facility [Member] | White Mountains Insurance Group Ltd. |  |  |  |  |  |  |  |  |  |  |  |  |
Cash flows from financing activities: |  |  |  |  |  |  |  |  |  |  |  |  |
Repayment of revolving line of credit |  |  |  |  |  |  |  |  |  | $ (75.0)us-gaap_RepaymentsOfDebt / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  |  |

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- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash -URI http://asc.fasb.org/extlink&oid=6506951 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 1 -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6676-107765 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3044-108585 + Details
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