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SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 2) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 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
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT |  |  |  |  |  |  |  |  |  |  |
Revenues (including realized gains and losses) | $ 689.7 us-gaap_Revenues | $ 589.4 us-gaap_Revenues | $ 652.8 us-gaap_Revenues | $ 578.3 us-gaap_Revenues | $ 626.6 us-gaap_Revenues | $ 574.1 us-gaap_Revenues | $ 489.3 us-gaap_Revenues | $ 627.4 us-gaap_Revenues | $ 2,510.2 us-gaap_Revenues | $ 2,317.4 us-gaap_Revenues | $ 2,435.7 us-gaap_Revenues
Expenses | 643.5 us-gaap_BenefitsLossesAndExpenses | 555.0 us-gaap_BenefitsLossesAndExpenses | 543.8 us-gaap_BenefitsLossesAndExpenses | 466.2 us-gaap_BenefitsLossesAndExpenses | 492.4 us-gaap_BenefitsLossesAndExpenses | 518.8 us-gaap_BenefitsLossesAndExpenses | 485.7 us-gaap_BenefitsLossesAndExpenses | 475.6 us-gaap_BenefitsLossesAndExpenses | 2,208.5 us-gaap_BenefitsLossesAndExpenses | 1,972.5 us-gaap_BenefitsLossesAndExpenses | 2,172.9 us-gaap_BenefitsLossesAndExpenses
Pre-tax loss | 46.2 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 34.4 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 109.0 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 112.1 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 134.2 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 55.3 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 3.6 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 151.8 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 301.7 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 344.9 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 262.8 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments
Income tax (expense) benefit | 8.7 us-gaap_IncomeTaxExpenseBenefit | (7.8)us-gaap_IncomeTaxExpenseBenefit | (23.9)us-gaap_IncomeTaxExpenseBenefit | (30.3)us-gaap_IncomeTaxExpenseBenefit | (27.4)us-gaap_IncomeTaxExpenseBenefit | (8.2)us-gaap_IncomeTaxExpenseBenefit | 0.6 us-gaap_IncomeTaxExpenseBenefit | (41.6)us-gaap_IncomeTaxExpenseBenefit | (53.3)us-gaap_IncomeTaxExpenseBenefit | (76.6)us-gaap_IncomeTaxExpenseBenefit | 15.7 us-gaap_IncomeTaxExpenseBenefit
Net income from continuing operations | 54.9 us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest | 26.6 us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest | 85.1 us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest | 81.8 us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest | 106.8 us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest | 47.1 us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest | 4.2 us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest | 110.2 us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest | 248.4 us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest | 268.3 us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest | 278.5 us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest
Net loss from discontinued operations, net of tax | (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 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
Comprehensive income attributable to White Mountains’ common shareholders |  |  |  |  |  |  |  |  | 211.1 us-gaap_ComprehensiveIncomeNetOfTax | 242.0 us-gaap_ComprehensiveIncomeNetOfTax | 302.6 us-gaap_ComprehensiveIncomeNetOfTax
Computation of net income (loss) available to common shareholders: |  |  |  |  |  |  |  |  |  |  |
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
White Mountains Insurance Group Ltd. |  |  |  |  |  |  |  |  |  |  |
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT |  |  |  |  |  |  |  |  |  |  |
Revenues (including realized gains and losses) |  |  |  |  |  |  |  |  | 0.3 us-gaap_Revenues / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 1.2 us-gaap_Revenues / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 20.8 us-gaap_Revenues / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Expenses |  |  |  |  |  |  |  |  | 32.3 us-gaap_BenefitsLossesAndExpenses / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 39.0 us-gaap_BenefitsLossesAndExpenses / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 32.4 us-gaap_BenefitsLossesAndExpenses / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Pre-tax loss |  |  |  |  |  |  |  |  | (32.0)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (37.8)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (11.6)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Income tax (expense) benefit |  |  |  |  |  |  |  |  | 0.9 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (0.1)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (0.3)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Net income from continuing operations |  |  |  |  |  |  |  |  | (31.1)us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (37.9)us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (11.9)us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Net loss from discontinued operations, net of tax |  |  |  |  |  |  |  |  | 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
Equity in earnings (losses) from consolidated and unconsolidated affiliates |  |  |  |  |  |  |  |  | 329.9 wtm_EquityInEarningsLossFromConsolidatedAndUnconsolidatedAffiliates / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 359.7 wtm_EquityInEarningsLossFromConsolidatedAndUnconsolidatedAffiliates / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 219.3 wtm_EquityInEarningsLossFromConsolidatedAndUnconsolidatedAffiliates / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
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
Other comprehensive income (loss), after-tax |  |  |  |  |  |  |  |  | (101.6)us-gaap_OtherComprehensiveIncomeLossNetOfTax / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | (79.8)us-gaap_OtherComprehensiveIncomeLossNetOfTax / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 95.2 us-gaap_OtherComprehensiveIncomeLossNetOfTax / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Comprehensive income attributable to White Mountains’ common shareholders |  |  |  |  |  |  |  |  | 211.1 us-gaap_ComprehensiveIncomeNetOfTax / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 242.0 us-gaap_ComprehensiveIncomeNetOfTax / dei_LegalEntityAxis = us-gaap_ParentCompanyMember | 302.6 us-gaap_ComprehensiveIncomeNetOfTax / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Computation of net income (loss) available to common shareholders: |  |  |  |  |  |  |  |  |  |  |
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

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