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Employee Share-Based Incentive Compensation Plans (OneBeason Peformance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Expense Amortized |  |  |
Ending December 31, | $ 184.6 wtm_IncentiveCompensationPayable | $ 218.3 wtm_IncentiveCompensationPayable |
Long Term Incentive Plan |  |  |
Expense Amortized |  |  |
Percentage of payout for minimum growth target percentage |  |  | 138.60%wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardPayoutForMinimumGrowthTargetPercentage / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember
Performance Shares |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number | 125,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options Outstanding Amended, Exercise Price | $ 742 wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingAmendedExercisePrice / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |
OneBeacon | Performance Shares | Long Term Incentive Plan |  |  |
Target options outstanding |  |  |
Beginning of period | 493,421 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 563,190 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 642,667 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember
Shares paid or expired (in shares) | (142,138)wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsTargetSharesPaidInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | (238,658)wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsTargetSharesPaidInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | (258,901)wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsTargetSharesPaidInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember
New grants | 165,800 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 179,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 181,290 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember
Assumed forfeitures and cancellations (in shares) | 387 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | (10,111)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | (1,866)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember
Ending December 31, | 517,470 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 493,421 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 563,190 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember
Expense Amortized |  |  |
Beginning of period | 4.0 wtm_IncentiveCompensationPayable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 1.2 wtm_IncentiveCompensationPayable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 9.7 wtm_IncentiveCompensationPayable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember
Payments and deferrals | (1.0)wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAmountsPaidInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 0 wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAmountsPaidInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | (7.7)wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAmountsPaidInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember
Assumed forfeitures and cancellations | 0 wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsForfeitedInPeriodTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | (0.1)wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsForfeitedInPeriodTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 0 wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsForfeitedInPeriodTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember
Expense recognized | 0.4 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | 2.9 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | (0.8)us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember
Ending December 31, | $ 3.4 wtm_IncentiveCompensationPayable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | $ 4.0 wtm_IncentiveCompensationPayable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember | $ 1.2 wtm_IncentiveCompensationPayable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember
Range of Performance Cycle from 2011 to 2013 [Member] | Long Term Incentive Plan |  |  |
Expense Amortized |  |  |
Percentage of payout for minimum growth target percentage | 37.10%wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardPayoutForMinimumGrowthTargetPercentage / us-gaap_PlanNameAxis = wtm_LongTermIncentivePlanMember / wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis = wtm_RangeOfPerformanceCycleFrom2011To2013Member |  |

X
- 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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Balance Type: | debit
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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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Balance Type: | na
Period Type: | duration

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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target 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 -Subparagraph (c)(2)(iii)(1) -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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Balance Type: | na
Period Type: | instant

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- Definition Number of options outstanding, including both vested and non-vested options. + 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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Balance Type: | na
Period Type: | instant

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- 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 Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements. + References No definition available. + Details
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- Definition The amount of performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + References No definition available. + Details
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- Definition The number of target performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + References No definition available. + Details
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- Definition This element represents the amended exercise price for the non-qualified options. + References No definition available. + Details
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Namespace Prefix: | wtm_
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- Definition Represents the lpercentage of target achieved, low end of range. + References No definition available. + Details
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- Details
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