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Third Party Reinsurance (Sirius Group) (Details) | 12 Months Ended |  | 12 Months Ended | 1 Months Ended | 12 Months Ended | 23 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended |  | 12 Months Ended
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Dec. 31, 2012 USD ($) | Dec. 31, 2011 USD ($) | Dec. 31, 2010 USD ($) | Dec. 31, 2009 USD ($) | Dec. 31, 2012 Sirius Group USD ($) | Dec. 31, 2011 Sirius Group USD ($) | Dec. 31, 2010 Sirius Group USD ($) | Dec. 31, 2012 Sirius Group USD ($) | Dec. 31, 2011 Sirius Group USD ($) | Dec. 31, 2010 Sirius Group USD ($) | Dec. 31, 2012 OneBeacon USD ($) Rating | Dec. 31, 2011 OneBeacon USD ($) | Dec. 31, 2010 OneBeacon USD ($) | Apr. 30, 2012 Property Catastrophe Reinsurance Program Sirius Group USD ($) | Dec. 31, 2012 Property Catastrophe Reinsurance Program Sirius Group USD ($) | Dec. 31, 2012 Property Catastrophe Reinsurance Program Sirius Group USD ($) | Apr. 30, 2010 New Zealand Earthquake Loss Cover Sirius Group USD ($) | Dec. 31, 2012 Nonproportional Aviation Portfolio Reinsurance Sirius Group USD ($) | Dec. 31, 2012 Minimum Sirius Group USD ($) | Apr. 30, 2011 Minimum Property Catastrophe Reinsurance Program Sirius Group USD ($) | Dec. 31, 2012 Minimum Facultative and Proportional Aviation Portfolio Reinsuranc Sirius Group USD ($) | Dec. 31, 2012 Minimum Nonproportional Aviation Portfolio Reinsurance Sirius Group USD ($) | Dec. 31, 2012 Minimum Personal Accident and Life Protection Sirius Group USD ($) | Dec. 31, 2012 Minimum Personal Accident and Life Protection Sirius Group EUR (€) | Dec. 31, 2012 Amount in excess of minimum [Member] Sirius Group USD ($) | Dec. 31, 2012 Amount in excess of minimum [Member] Property Catastrophe Reinsurance Program Sirius Group USD ($) | Dec. 31, 2012 Amount in excess of minimum [Member] Personal Accident and Life Protection Sirius Group USD ($) | Dec. 31, 2012 Amount in excess of minimum [Member] Personal Accident and Life Protection Sirius Group EUR (€) | Dec. 31, 2012 Maximum Facultative and Proportional Aviation Portfolio Reinsuranc Sirius Group USD ($) | Dec. 31, 2012 Maximum Nonproportional Aviation Portfolio Reinsurance Sirius Group USD ($) | Dec. 31, 2012 Tokio Marine and Nichido Fire | Dec. 31, 2012 General Reinsurance Corporation Sirius Group USD ($) | Dec. 31, 2012 Swiss Re Group Sirius Group USD ($) | Dec. 31, 2012 Olympus Sirius Group USD ($) | Dec. 31, 2012 Lloyds of London Sirius Group USD ($) | Dec. 31, 2012 Michigan Catastorphic Claims Association Sirius Group USD ($) | Dec. 31, 2012 Non US and Non Japan Sirius Group USD ($) | Dec. 31, 2012 New Madrid Sirius Group USD ($) | Dec. 31, 2012 European Windstorms and Flood Sirius Group USD ($) | Dec. 31, 2012 London Branch Minimum Sirius Group USD ($) | Dec. 31, 2012 London Branch Amount in excess of minimum [Member] Sirius Group USD ($) | Dec. 31, 2012 Hurricane Sandy [Member] Sirius Group USD ($)
Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
AM best rating system number of ratings |  |  |  |  |  |  |  |  |  |  | 16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss reinsurance protection, amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5,000,000 |  |  |  |  |  | $ 15,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Reinsurance Protection Purchased for 2013 for the London Facultative branch |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 20,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Reinsurance Protection for the London branch for facultative and direct U.S.-catastrophe exposed business (excluding Florida risks) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance amount In excess of retention |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 300,000 |  |
Insurance recoveries |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,000,000
Reinsurance recoverable on paid and unpaid losses | 447,000,000 | 2,537,800,000 |  |  |  |  |  | 336,300,000 | 353,700,000 |  | 110,700,000 | 2,184,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 56,200,000 | 36,400,000 | 29,600,000 | [1] | 19,400,000 | [2] | 14,700,000 | [3] |  |  |  |  |  |
Reinsurance recoverable cover under reinsurance contract |  |  |  |  |  |  |  |  |  |  |  |  |  | 17,000,000 |  |  | 65,000,000 |  |  |  |  |  | 7,000,000 | 5,000,000 |  | 40,000,000 | 13,000,000 | 10,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance program reinsured losses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 150,000,000 |  |  |  |  |  |  |  | 2,250,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of insurance losses retained |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 35,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance program cover in first event industry loss warranties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10,000,000 |  | 5,000,000,000 |  |  |
Industry Loss Trigger for Reinsurance Program Cover in Earthquake Loss Warranties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7,500,000,000 |  |  |  |  |
Reinsurance program cover in second event industry loss warranties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,000,000 |  |  |  |  |
Reinsurance program cover in earthquake loss warranties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 37,500,000 |  |  |  |
Reinsurance program cover in first and second event industry loss warranties |  |  |  |  |  |  |  | 20,000,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance program market loss event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,000,000 |  |  |
Loss and loss adjustment expenses | 1,193,900,000 | 1,174,300,000 | 1,216,600,000 |  | 543,900,000 | 626,000,000 | 531,000,000 | 543,900,000 | 626,000,000 | 531,000,000 | 650,000,000 | 548,300,000 | 685,600,000 |  |  | 47,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of quota share reinsurance agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance coverage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 40,000,000 |  |  |
Percentage of trade credit business ceded under quota share retrocession |  |  |  |  | 20.00% |  |  | 20.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of bond business ceded under renewed quota share retrocession |  |  |  |  | 50.00% |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of Property Catastrophe Reinsurance Treaty Reinstatement Premiums |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on paid losses | 17,900,000 | 30,500,000 |  |  |  |  |  | 14,600,000 |  |  | 3,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on unpaid losses | 429,100,000 | 2,507,300,000 | 2,344,000,000 | 2,771,500,000 |  |  |  | 321,800,000 |  |  | 107,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of total reinsurance recoverables |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 17.00% | 11.00% | 9.00% | [1] | 6.00% | [2] | 4.00% | [3] |  |  |  |  |  |
Percentage collateralized |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.00% | 2.00% | 100.00% | [1] | 8.00% | [2] | 0.00% | [3] |  |  |  |  |  |
Seller note from acquisition | 25,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aviation reinsurance program first event industry loss warranties required industry losses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 33,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aviation reinsurance program cover in second event industry loss warranties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aviation reinsurance program cover in first event industry loss warranties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400,000,000 |  |  |  |  |  |  |  | 1,000,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance Coverage for Marine Yachat Portfolio |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 9,800,000 |
[1] | Non-U.S. insurance entity. The balance is fully collateralized through funds held, letters of credit or trust agreements.
[2] | Represents the total of reinsurance recoverables due to Sirius Group from all Lloyds Syndicates.
[3] | Michigan Catastrophic Claims Association (”MCCA”) is a non-profit unincorporated association., established by the State of Michigan with the power to issue and collect assessments, to which every insurance company that sells automobile coverage in Michigan is required to be a member. A.M. Best does not rate MCCA. Sirius Group acquired its recoverable from MCCA in the acquisition of Stockbridge Insurance Company. As part of the acquisition, Sirius Group obtained $25.0 of reinsurance protection from the seller (currently rated A+ by A.M. Best) for unfavorable loss reserve development, including uncollectible reinsurance.

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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 Additional Reinsurance Coverage under European ILW + References No definition available. + Details
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- Definition Additional Reinsurance Protection for the London branch for facultative and direct U.S.-catastrophe exposed business and direct U.S.-catastrophe exposed business (excluding Florida risks) + References No definition available. + Details
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- Definition Additional Reinsurance Protection Purchased for 2013 for the London Facultative branch + References No definition available. + Details
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- Definition Additional Reinsurance Protection for the London branch for facultative + 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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- Definition Represents the cover which the entity has purchased in case of first event industry loss warranties under the aviation reinsurance program. + References No definition available. + Details
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- Definition Represents the cover which the entity has purchased in case of second event industry loss warranties under the aviation reinsurance program. + References No definition available. + Details
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- Definition Represents the amount of aggregate industry losses required for availing the cover which the entity has purchased in case of first event industry loss warranties under the aviation reinsurance program. + References No definition available. + Details
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- Definition Industry Loss Trigger for Reinsurance Program Cover in Earthquake Loss Warranties + References No definition available. + Details
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- Definition Insurance Amount In Excess of Retention + References No definition available. + Details
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- Definition Represents the percentage of a top reinsurer's recoverable amounts that are collateralized. + References No definition available. + Details
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- Definition Represents the percentage of ceded bond business under a renewed quota share retrocession. + References No definition available. + Details
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- Definition Percentage of Property Catastrophe Reinsurance Treaty Reinstatement Premiums + References No definition available. + Details
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- Definition Represents the percentage ceded under quota share reinsurance agreement. + 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 Represents the percentage of ceded trade credit business under a quota share retrocession. + References No definition available. + Details
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- Definition Reinsurance Coverage for Marine Yacht Portfolio + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Reinsurance Coverage Program European Storm ILW + References No definition available. + Details
Name: | wtm_ReinsuranceCoverageProgramEuropeanStormIlw
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | wtm_ReinsuranceLineItems
Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Reinsurance Program Cover in Earthquake Loss Warranties + References No definition available. + Details
Name: | wtm_ReinsuranceProgramCoverInEarthquakeLossWarranties
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Reinsurance Program Cover in Second Event Industry Loss Warranties + References No definition available. + Details
Name: | wtm_ReinsuranceProgramCoverInSecondEventIndustryLossWarranties
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the reinsured losses under reinsurance program. + References No definition available. + Details
Name: | wtm_ReinsuranceProgramReinsuredLosses
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 Reinsurance Recoverable on Cat Reinsurance + References No definition available. + Details
Name: | wtm_ReinsuranceRecoverableOnCatReinsurance
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Seller note from acquisition of Stockbridge. + References No definition available. + Details
Name: | wtm_SellerNoteFromAcquisition
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration