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v2.4.1.9

Summary of Significant Accounting Policies (Insurance and Reinsurance Operations) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Workers compensation discount percent | 2.50%us-gaap_WorkersCompensationDiscountPercent | 3.50%us-gaap_WorkersCompensationDiscountPercent
Workers compensation discount amount | $ 1.0 us-gaap_WorkersCompensationDiscountAmount | $ 3.0 us-gaap_WorkersCompensationDiscountAmount
OneBeacon Runoff |  |
Workers compensation discount amount |  | $ 61.7 us-gaap_WorkersCompensationDiscountAmount / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = wtm_OnebeaconRunoffMember

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- Definition Current year monetary amount of the change in worker's compensation reserve liability resulting from the discount applied to reduce the reserve to present value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph b -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),13(b)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Percent discount applied to worker's compensation reserve liability to reduce the reserve to present value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph b -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.13(b)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6482852&loc=d3e14931-158439 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14754-158437 + Details
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- Details
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