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Third-Party Reinsurance (OneBeacon) (Details) (USD $) |  |  |  |  | 12 Months Ended |  |  | 12 Months Ended | 1 Months Ended | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2012 OneBeacon Rating | Dec. 31, 2011 OneBeacon | Dec. 31, 2012 Maximum OneBeacon | Dec. 31, 2012 General Reinsurance Corporation Sirius Group | Dec. 31, 2012 OneBeacon Property Catastrophe Reinsurance Program | Dec. 31, 2011 OneBeacon Property Catastrophe Reinsurance Program | Dec. 31, 2012 OneBeacon Property Catastrophe Reinsurance Program Maximum | Dec. 31, 2012 OneBeacon Property Catastrophe Reinsurance Program Minimum | Dec. 31, 2012 OneBeacon Collector cars and boats property cat treaty [Member] | Dec. 31, 2012 OneBeacon Collector cars and boats property cat treaty [Member] Maximum | Dec. 31, 2012 OneBeacon Collector cars and boats property cat treaty [Member] Amount in excess of minimum [Member] | Dec. 31, 2012 OneBeacon Collector cars and boats property cat treaty [Member] Minimum | Dec. 31, 2012 OneBeacon IMU Treaty | Dec. 31, 2012 OneBeacon IMU Treaty Maximum | Dec. 31, 2012 OneBeacon IMU Treaty Amount in excess of minimum [Member] | Dec. 31, 2012 OneBeacon IMU Treaty Minimum | Jun. 30, 2010 OneBeacon Northeast Homeowners Business | Dec. 31, 2012 OneBeacon Property Per Risk Reinsurance Program | Dec. 31, 2012 OneBeacon Property Per Risk Reinsurance Program Maximum | Dec. 31, 2012 OneBeacon Property Per Risk Reinsurance Program Minimum | Dec. 31, 2012 OneBeacon Casualty Reinsurance Program | Dec. 31, 2012 OneBeacon Casualty Reinsurance Program Maximum | Dec. 31, 2012 OneBeacon Casualty Reinsurance Program Amount in excess of minimum [Member] | Dec. 31, 2012 OneBeacon Casualty Reinsurance Program Minimum | Dec. 31, 2012 OneBeacon Workers compensation [Member] | Dec. 31, 2012 OneBeacon Workers compensation [Member] Maximum | Dec. 31, 2012 OneBeacon Workers compensation [Member] Minimum | Dec. 31, 2012 OneBeacon Casualty Multiple Insured [Member] Amount in excess of minimum [Member] | Dec. 31, 2012 OneBeacon Casualty Multiple Insured [Member] Minimum | Dec. 31, 2012 OneBeacon Professional Liability [Member] | Dec. 31, 2012 OneBeacon Professional Liability [Member] Maximum | Dec. 31, 2012 OneBeacon Professional Liability [Member] Amount in excess of minimum [Member] | Dec. 31, 2012 OneBeacon Professional Liability [Member] Minimum | Dec. 31, 2012 Hurricane Sandy [Member] OneBeacon Property Catastrophe Reinsurance Program
Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
AM best rating system number of ratings |  |  |  |  | 16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on paid and unpaid losses | $ 447,000,000 | $ 2,537,800,000 |  |  | $ 110,700,000 | $ 2,184,100,000 |  | $ 56,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of total reinsurance recoverables |  |  |  |  |  |  |  | 17.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of certified losses required to be shared by federal government subject to terrorism risk insurance act |  |  |  |  | 85.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liability to be paid by government and industry subject to terrorism risk insurance act |  |  |  |  | 100,000,000,000 |  | 100,000,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property catastrophe reinsurance program first layer |  |  |  |  |  |  |  |  | 25,000,000 |  |  |  |  |  |  | 2,500,000 |  |  | 5,000,000 | 2,000,000 |  |  | 50,000,000 | 20,000,000 |  |  | 6,000,000 | 5,000,000 |  |  |  |  |  |  |  | 5,000,000 | 5,000,000 |
Property Catastrophe Reinsurance Treaty Second Layer Co-Partipation Percentage |  |  |  |  |  |  |  |  |  |  |  |  | 20.00% |  |  |  |  |  |  |  |  | 20.00% |  |  | 10.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Property catastrophe reinsurance program, second layer |  |  |  |  |  |  |  |  | 155,000,000 |  |  |  |  |  | 20,000,000 | 5,000,000 |  |  |  |  |  |  |  | 50,000,000 |  |  | 10,000,000 | 11,000,000 |  |  |  |  |  |  |  |  |  |
Property catastrophe reinsurance program second layer first retention percentage |  |  |  |  |  |  |  |  | 55.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property catastrophe reinsurance program first retention slab for second layer |  |  |  |  |  |  |  |  |  |  | 40,000,000 | 25,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property catastrophe reinsurance program second layer, second retention percentage |  |  |  |  |  |  |  |  | 15.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property catastrophe reinsurance program second retention slab for second layer |  |  |  |  |  |  |  |  |  |  | 80,000,000 | 40,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property catastrophe reinsurance program second layer third retention percentage |  |  |  |  |  |  |  |  | 10.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property catastrophe reinsurance program third retention slab for second layer |  |  |  |  |  |  |  |  |  |  | 180,000,000 | 80,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property catastrophe reinsurance retention |  |  |  |  |  |  |  |  | 49,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property catastrophe reinsurance program third layer |  |  |  |  |  |  |  |  |  | 225,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Losses Ceded, Hurricane |  |  |  |  |  |  |  |  |  |  |  |  | 11,900,000 |  |  |  | 41,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15,600,000
Property catastrophe reinsurance treaty loss coverage |  |  |  |  |  |  |  |  |  |  |  |  |  | 25,000,000 | 2,500,000 |  |  | 60,000,000 |  |  |  |  |  |  |  |  |  |  |  | 10,000,000 | 1,000,000 |  |  |  |  |  |  |
Property catastrophe treaty aggregate deductible for large losses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property Catastrophe Treaty Aggregate Deductible |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property catastrophe reinsurance treaty half loss layer |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10,000,000 | 50,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property catastrophe reinsurance treaty first layer co-partipation percentage |  |  |  |  |  |  |  |  |  |  |  |  | 5.00% |  |  |  |  |  |  |  |  | 10.00% |  |  | 22.50% |  |  |  |  |  |  |  |  | 20.00% |  |  |  |
Workers compensation treaty coverage percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |
Property catastrophe reinsurance treaty reinstatement premiums |  |  |  |  |  |  |  |  |  |  |  |  | 1,400,000 |  |  |  | 5,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,900,000
Property catastrophe treaty per occurance amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 60,000,000 |  | 2,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated individual retention level for commercial plicies, next fiscal year |  |  |  |  | 100,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ceded premiums written under quota share reinsurance agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance program reinsured terrorism losses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100,000,000 | 10,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance program reinsured losses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 21,000,000 |  | 5,000,000 |  |  |  |  |  |  | 20,000,000 |  | 5,000,000 |
Reinsurance recoverable cover under reinsurance contract |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 40,000,000 |  |  |  |  |  |
Amount of insurance losses retained |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10,000,000 |  |  |  |  |
Reinsurance recoverable on paid losses | 17,900,000 | 30,500,000 |  |  | 3,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on unpaid losses | 429,100,000 | 2,507,300,000 | 2,344,000,000 | 2,771,500,000 | 107,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverables on unpaid losses |  |  |  |  | 90.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Largest recoverable from reinsurer |  |  |  |  | $ 10,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition The portion of reinsurance receivables representing the aggregate amount of estimated amounts due as of the balance sheet date from another insurer; also called reinsurance recoverables or unpaid losses recoverable, that are expected based upon statistical projections of incurred claims and claims settlement expenses which have not yet been reported to the insurer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 14, 15 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489 + Details
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- Definition Receivables currently due from reinsurers for ceded claims paid. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489 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 210.7-03.6) -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 605 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 14, 15 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 + Details
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- Definition The known and estimated amount recoverable as of the balance sheet date from reinsurers for claims paid or incurred by the ceding insurer and associated claims settlement expenses, including estimated amounts for claims incurred but not reported, and policy benefits, net of any related valuation allowance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 14, 16, 27, 113 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387 Reference 4: 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),6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition The number of ratings in the A.M. Best rating system. + References No definition available. + Details
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- Definition Represents the amount of exposure to losses resulting from the insurance claims retained by the entity. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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- Definition Total consideration paid and payable to reinsurers during the period pursuant to terms of quota share reinsurance arrangements. + References No definition available. + Details
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- Definition Represents the estimated individual retention level for commercial policies subject to the Terrorism Act in the next fiscal year based on net written premiums in the current fiscal year. + References No definition available. + Details
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- Definition Incurred Losses Ceded, Hurricane + References No definition available. + Details
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- Definition Largest Recoverable from reinsurer + References No definition available. + Details
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- Definition Represents the aggregate liability to be paid by the government and industry without further action by Congress. + References No definition available. + Details
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Balance Type: | credit
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- Definition Represents the percentage of certified losses required to be shared by federal government once a company meets certain conditions. + References No definition available. + Details
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- Definition The respective third party reinsurers' percentage share of the total paid and unpaid reinsurance recoverables. + References No definition available. + Details
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- Definition First layer of reinsurance agreement that is retained by the entity. + References No definition available. + Details
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- Definition Represents the retention for the second layer of the reinsurance agreement. + References No definition available. + Details
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- Definition Second layer of reinsurance agreement, in excess of the first layer, that is reinsured. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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- Definition Represents the retention percentage between $25 million and $40 million for the second layer of the reinsurance agreement. + References No definition available. + Details
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- Definition Represents the retention percentage between $40 million and $80 million for the second layer of the reinsurance agreement. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
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- Definition Represents the retention percentage between $80 million and $180 million for the second layer of the reinsurance agreement. + References No definition available. + Details
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Data Type: | num:percentItemType
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- Definition Represents the retention for the second layer of the reinsurance agreement. + References No definition available. + Details
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- Definition Third layer of reinsurance agreement, in excess of the second layer, that is retained by the entity. + References No definition available. + Details
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Balance Type: | debit
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- Definition Represents the retention for the second layer of the reinsurance agreement. + References No definition available. + Details
Name: | wtm_PropertyCatastropheReinsuranceProgramThirdRetentionSlabForSecondLayer
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Data Type: | xbrli:monetaryItemType
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- Definition Property Catastrophe Reinsurance Retention + References No definition available. + Details
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Balance Type: | debit
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- Definition Property Catastrophe Reinsurance Treaty First Layer Co-Partipation Percentage + References No definition available. + Details
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Data Type: | num:percentItemType
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- Definition Property Catastrophe Reinsurance Treaty Half Loss Layer + References No definition available. + Details
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- Definition Property Catastrophe Reinsurance Treaty Loss Coverage + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Property Catastrophe Reinsurance Treaty Reinstatement Premiums + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Property Catastrophe Reinsurance Treaty Second Layer Co-Partipation Percentage + References No definition available. + Details
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Data Type: | num:percentItemType
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- Definition Property Catastrophe Treaty Aggregate Deductible + References No definition available. + Details
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Balance Type: | debit
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- Definition Property Catastrophe Treaty Aggregate Deductible for Large Losses + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Property Catastrophe Treaty Per Occurance Amount + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Details
Name: | wtm_ReinsuranceLineItems
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the reinsured losses under reinsurance program. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the reinsurance protection on an individual risk basis for terrorism losses under reinsurance program. + References No definition available. + Details
Name: | wtm_ReinsuranceProgramReinsuredTerrorismLosses
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the cover amount under reinsurance contracts. + References No definition available. + Details
Name: | wtm_ReinsuranceRecoverableCoverUnderReinsuranceContract
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Percentage of Reinsurance Recoverables on Unpaid Losses Rated A or Better + References No definition available. + Details
Name: | wtm_ReinsuranceRecoverablesOnUnpaidLosses
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Workers Compensation Treaty Coverage Percentage + References No definition available. + Details
Name: | wtm_WorkersCompensationTreatyCoveragePercentage
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration