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Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Discontinued Operations [Line Items} |  |  |
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax | $ 8.0 us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax | $ 0 us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax |
Net loss from discontinued operations, net of tax | (7.9)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax | (0.5)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax |
Other revenue | 35.8 us-gaap_OtherIncome | (3.4)us-gaap_OtherIncome | (3.4)us-gaap_OtherIncome
Segment, Discontinued Operations |  |  |
Discontinued Operations [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 |  |
Net loss from discontinued operations, net of tax | (0.1)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember | (0.5)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax / us-gaap_StatementOperatingActivitiesSegmentAxis = us-gaap_SegmentDiscontinuedOperationsMember |
OneBeacon Runoff [Member] | Segment, Discontinued Operations |  |  |
Discontinued Operations [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 |

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- 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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- Definition Amount of income (loss) from a disposal group, net of income tax before extraordinary items allocable to noncontrolling interests. Includes, net of tax, income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal. + 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 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.12) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 13 -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.14) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 15 -Article 5 + Details
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