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Third Party Reinsurance (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2010
Reinsurance [Line Items] |  |  |  |  |  |
Reinsurance recoverable on paid losses | $ 14.8 |  | $ 30.5 |  |  |
Reinsurance recoverable on unpaid losses | 356.8 | 2,369.4 | 2,507.3 | 2,581.2 | 2,193.0 | 2,344.0
Purchase accounting adjustments | 153 |  |  |  |  |
Amount Retained Under Reinsurance Cover | 49.3 |  |  |  |  |
OneBeacon |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |
Number of ratings in A.M. Best ratings | 15 |  |  |  |  |
Reinsurance recoverable on paid losses | 2.0 |  |  |  |  |
Reinsurance recoverable on unpaid losses | 41.3 |  |  |  |  |
Reinsurance recoverables | 1,449.2 |  |  |  |  |
Number of Contracts Seller Caused One Beacon to Purchase Prior to Acquisition | 2 |  |  |  |  |
OneBeacon | Property catastrophe reinsurance program |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |
Amount of loss on a single catastrophe retained, first layer | 25.0 |  |  |  |  |
Amount of loss on a single catastrophe reinsured, second layer | 155.0 |  |  |  |  |
Property catastrophe reinsurance program second layer first retention percentage | 55.00% |  |  |  |  |
Second retention percentage of loss on a single catastrophe reinsured, second layer | 15.00% |  |  |  |  |
Property catastrophe reinsurance program second layer third retention percentage | 10.00% |  |  |  |  |
Amount of loss on a single catastrophe retained, third layer | 180.0 |  |  |  |  |
OneBeacon | Property catastrophe reinsurance program | Minimum |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |
Amount of loss on a single catastrophe reinsured, first retention slab for second layer | 25.0 |  |  |  |  |
Amount of loss on a single catastrophe reinsured, second retention slab for second layer | 40.0 |  |  |  |  |
Property catastrophe reinsurance program third retention slab for second layer | 80.0 |  |  |  |  |
OneBeacon | Property catastrophe reinsurance program | Maximum |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |
Amount of loss on a single catastrophe reinsured, first retention slab for second layer | 40.0 |  |  |  |  |
Amount of loss on a single catastrophe reinsured, second retention slab for second layer | 80.0 |  |  |  |  |
Property catastrophe reinsurance program third retention slab for second layer | 180.0 |  |  |  |  |
OneBeacon | Hannover Ruversich [Member] |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |
Reinsurance recoverables | 6.4 |  |  |  |  |
Percentage of total paid and unpaid reinsurance recoverables | 15.00% |  |  |  |  |
OneBeacon | Hartford Steam Boiler [Member] |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |
Reinsurance recoverables | 4.6 |  |  |  |  |
Percentage of total paid and unpaid reinsurance recoverables | 11.00% |  |  |  |  |
OneBeacon | Munich Reinsurance America |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |
Reinsurance recoverables | 4.5 |  |  |  |  |
Percentage of total paid and unpaid reinsurance recoverables | 10.00% |  |  |  |  |
OneBeacon | Platinum Underwriters Re [Member] |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |
Reinsurance recoverables | 4.1 |  |  |  |  |
Percentage of total paid and unpaid reinsurance recoverables | 9.00% |  |  |  |  |
OneBeacon | Transatlantic Reinsurance [Member] |  |  |  |  |  |
Reinsurance [Line Items] |  |  |  |  |  |
Reinsurance recoverables | $ 2.0 |  |  |  |  |
Percentage of total paid and unpaid reinsurance recoverables | 5.00% |  |  |  |  |

X
- 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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X
- 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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X
- 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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X
- Definition Amount retained under reinsurance cover in an event in a $180 million loss. + References No definition available. + Details
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X
- Definition Represents the number of reinsurance contracts the seller caused OneBeacon to purchase from subsidiaries of Berkshire Hathaway Inc. immediately prior to the entity's acquisition of OneBeacon. + References No definition available. + Details
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Data Type: | xbrli:integerItemType
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X
- 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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X
- Definition First layer of reinsurance agreement that is retained by the entity. + References No definition available. + Details
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Balance Type: | debit
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X
- Definition Represents the retention for the second layer of the reinsurance agreement. + References No definition available. + Details
Name: | wtm_PropertyCatastropheReinsuranceProgramFirstRetentionSlabForSecondLayer
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Second layer of reinsurance agreement, in excess of the first layer, that is reinsured. + References No definition available. + Details
Name: | wtm_PropertyCatastropheReinsuranceProgramSecondLayer
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the retention percentage between $40 million and $80 million for the second layer of the reinsurance agreement. + References No definition available. + Details
Name: | wtm_PropertyCatastropheReinsuranceProgramSecondLayerSecondRetentionPercentage
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- 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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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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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
Name: | wtm_PropertyCatastropheReinsuranceProgramThirdLayer
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the retention for the second layer of the reinsurance agreement. + References No definition available. + Details
Name: | wtm_PropertyCatastropheReinsuranceProgramThirdRetentionSlabForSecondLayer
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 Purchase accounting adjustments related to reinsurance recoverables. + References No definition available. + Details
Name: | wtm_ReinsuranceRecoverablesGrossPurchaseAccountingAdjustments
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant