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Contingencies (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 1 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Oct. 31, 2011 | Jun. 30, 2007 | Oct. 07, 2011 | Jan. 22, 2015
Loss Contingencies [Line Items] |  |  |  |  |  |
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax | $ 8.0 us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax | $ 0 us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax |  |  |  |
Esurance |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Warranty agreement period |  |  | 18 months |  |  |
Non-solicitation agreement period |  |  | 3 years |  |  |
Percentage of fluctuation in loss reserves of transferred subsidiaries retained |  |  |  |  | 90.00%wtm_PercentageOfFluctuationInLossReservesOfTransferredSubsidiariesRetained / dei_LegalEntityAxis = wtm_EsuranceMember |
Tribune Company Litigation | Pending Litigation |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Proceeds from Sale, Maturity and Collection of Investments |  |  |  | 39.0 wtm_ProceedsFromTribuneCommonStock / us-gaap_LitigationCaseAxis = wtm_TribuneCompanyLitigationMember / us-gaap_LitigationStatusAxis = us-gaap_PendingLitigationMember |  |
Segment, Discontinued Operations |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax | 8.0 us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |  |  |
OneBeacon Runoff [Member] |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Number of insurance holders |  |  |  |  |  | 3 wtm_Numberofinsuranceholders / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember
OneBeacon Runoff [Member] | Segment, Discontinued Operations |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax | $ (0.1)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember | $ (0.5)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |  |  |  |

X
- Definition Amount of gain (loss), after tax expense or benefit and not previously recognized, resulting from the sale of a business component. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e957-107759 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760 + Details
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- Details
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- Definition Non-Soliciation Agreement Period + References No definition available. + Details
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Balance Type: | na
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- Definition Number of insurance holders + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percentage of favorable or unfavorable loss reserve development in the Transferred Subsidiaries retained by the entity. + References No definition available. + Details
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Balance Type: | na
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- Definition Proceeds from Tribune common stock tendered in connecttion with Tribune's leveraged buyout. + References No definition available. + Details
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Namespace Prefix: | wtm_
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Balance Type: | credit
Period Type: | duration

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- Definition Warranty Agreement Period + References No definition available. + Details
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- Details
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- Details
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X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Balance Type: |
Period Type: |