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Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) (White Mountains, W T M Incentive Plan, USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Restricted Share Cliff Vesting in January 2018 |  |
Target Restricted Share Activity |  |
Issued (in shares) | 22,890 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_AwardTypeAxis = wtm_RestrictedShareCliffVestinginJanuary2018Member / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember |
Restricted Share Cliff Vesting in January 2017 |  |
Target Restricted Share Activity |  |
Issued (in shares) | 20,400 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_AwardTypeAxis = wtm_RestrictedShareCliffVestinginJanuary2017Member / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember |
Restricted Stock |  |
Target Restricted Share Activity |  |
Beginning of period (in shares) | 83,314 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | 94,130 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember
Issued (in shares) | 22,890 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | 20,400 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember
Vested (in shares) | (35,079)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | (33,205)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember
Assumed forfeitures and cancellations | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember
Ending of period (in shares) | 71,125 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | 81,325 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember
Unamortized grant date fair value |  |
Unamortized grant date fair value, beginning balance | $ 14.3 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | $ 17.0 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember
Issued | 15.2 wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsIssuedInPeriodTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | 11.8 wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsIssuedInPeriodTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember
Assumed forfeitures and cancellations | 0 wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsForfeitedInPeriodTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | 0 wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsForfeitedInPeriodTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember
Expense recognized | (3.0)us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | (4.0)us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember
Unamortized grant date fair value, ending balance | $ 26.5 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember | $ 24.8 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / dei_LegalEntityAxis = wtm_WhiteMountainsMember / us-gaap_PlanNameAxis = wtm_WTMIncentivePlanMember

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Number of share instruments newly issued under a share-based compensation plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The total reduction to unamortized compensation cost for performance shares forfeited during the period. + References No definition available. + Details
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- Definition The total fair value (on the date of issue) for restricted share awards issued during the period. + References No definition available. + Details
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