XML 153 R137.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Employee Share-Based Incentive Compensation Plans (Under Qualified Retirement Plans) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
One Beacon Employee Stock Ownership Plan |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Defined Contribution Plan, Cost Recognized | $ 4.4 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = wtm_OneBeaconEmployeeStockOwnershipPlanMember | $ 6.3 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = wtm_OneBeaconEmployeeStockOwnershipPlanMember | $ 4.8 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = wtm_OneBeaconEmployeeStockOwnershipPlanMember
Qualified Plan [Member] |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Eligible contribution percentage | 2.00%wtm_DeferredCompensationArrangementVariableContributionByEligibleParticipantsAsPercentageOfSalary / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | 4.00%wtm_DeferredCompensationArrangementVariableContributionByEligibleParticipantsAsPercentageOfSalary / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | 3.00%wtm_DeferredCompensationArrangementVariableContributionByEligibleParticipantsAsPercentageOfSalary / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Percentage of common shares outstanding | 3.00%wtm_PercentageOfCommonSharesOutstandingOwnedByPlans / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | 3.00%wtm_PercentageOfCommonSharesOutstandingOwnedByPlans / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |

X
- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
Name: | us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

X
- Definition Represents the variable contribution amounts earned by eligible participants of the KSOP as a percentage of salary. + References No definition available. + Details
Name: | wtm_DeferredCompensationArrangementVariableContributionByEligibleParticipantsAsPercentageOfSalary
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the percentage of total common shares outstanding owned by the plans. + References No definition available. + Details
Name: | wtm_PercentageOfCommonSharesOutstandingOwnedByPlans
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis=wtm_OneBeaconEmployeeStockOwnershipPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |