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Third Party Reinsurance (Details) (USD $) |  |  |  |  |  | 3 Months Ended | 130 Months Ended | 3 Months Ended |  | 3 Months Ended |  | 3 Months Ended |  |  |
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Mar. 31, 2012 | Dec. 31, 2011 | Mar. 31, 2011 | Dec. 31, 2010 | Mar. 31, 2012 OneBeacon contract rating | Mar. 31, 2012 OneBeacon Property catastrophe reinsurance program | Mar. 31, 2012 OneBeacon Property catastrophe reinsurance program Minimum | Mar. 31, 2012 OneBeacon Property catastrophe reinsurance program Maximum | Mar. 31, 2012 OneBeacon NICO Cover | Mar. 31, 2012 OneBeacon NICO Cover Maximum | Mar. 31, 2012 OneBeacon GRC Cover | Mar. 31, 2012 OneBeacon GRC Cover Maximum | Mar. 31, 2012 OneBeacon National Indemnity Company and General Reinsurance Corporation | Mar. 31, 2012 OneBeacon Hanover Insurance Group Ltd. | Mar. 31, 2012 OneBeacon Tokio Marine and Nichido Fire | Mar. 31, 2012 OneBeacon Tower Group Inc. | Mar. 31, 2012 OneBeacon Munich Reinsurance America | May 31, 2001 OneBeacon General Reinsurance Corporation GRC Cover | Mar. 31, 2012 Sirius group rating | Mar. 31, 2012 Sirius group General Reinsurance Corporation | Mar. 31, 2012 Sirius group Swiss Re Group | Mar. 31, 2012 Sirius group Olympus | Mar. 31, 2012 Sirius group Lloyds of London | Mar. 31, 2012 Sirius group General Insurance Company of India
Third-Party Reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on paid losses | $ 39,100,000 | $ 30,500,000 |  |  | $ 25,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 13,200,000 |  |  |  |  |
Reinsurance recoverable on unpaid losses | 2,465,400,000 | 2,507,300,000 | 2,304,100,000 | 2,344,000,000 | 2,269,000,000 |  |  |  |  |  | 423,700,000 |  |  |  |  |  |  |  | 356,400,000 |  |  |  |  |
Purchase accounting adjustments |  |  |  |  | 160,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverables |  |  |  |  |  |  |  |  |  |  |  |  | 1,504,900,000 | 79,900,000 | 55,000,000 | 29,900,000 | 24,000,000 |  |  | 42,600,000 | 41,800,000 | 33,300,000 | 26,800,000 | 14,800,000
Percentage of total paid and unpaid reinsurance recoverables |  |  |  |  |  |  |  |  |  |  |  |  | 66.00% | 3.00% | 2.00% | 1.00% | 1.00% |  |  | 12.00% | 11.00% | 9.00% | 7.00% | 4.00%
Percentage of reinsurance recoverable that is collateralized |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.00% | 4.00% | 100.00% | 9.00% | 2.00%
Reinsurance recoverable from various third-party reinsurers, guaranteed by the third-party reinsurer under reinsurance agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 29,000,000 |  |  |  |  |  |  |  |  |
Number of reinsurance contracts the seller caused OneBeacon to purchase immediately prior to the entity's acquisition of OneBeacon |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cover under reinsurance contract |  |  |  |  |  |  |  |  |  | 2,500,000,000 |  | 570,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Adverse loss reserve development on losses occurring in years 2000 and prior |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400,000,000 |  |  |  |  |  |
Reserves ceded as of the date of the OneBeacon acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 170,000,000 |  |  |  |  |  |
Reinsurance receivables paid by the reinsurer |  |  |  |  |  |  |  |  | 2,300,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of loss on a single catastrophe retained, first layer |  |  |  |  |  | 50,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of loss on a single catastrophe reinsured, second layer |  |  |  |  |  | 175,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
First retention percentage of loss on a single catastrophe reinsured, second layer |  |  |  |  |  | 26.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of loss on a single catastrophe reinsured, first retention slab for second layer |  |  |  |  |  |  | 50,000,000 | 100,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Second retention percentage of loss on a single catastrophe reinsured, second layer |  |  |  |  |  | 10.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of loss on a single catastrophe reinsured, second retention slab for second layer |  |  |  |  |  |  | 100,000,000 | 175,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of loss on a single catastrophe retained, third layer |  |  |  |  |  | $ 225,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
The percentage ceded under quota share reinsurance agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |
Number of ratings in A.M. Best ratings |  |  |  |  | 15 |  |  |  |  |  |  |  |  |  |  |  |  |  | 15 |  |  |  |  |

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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 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 SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 6 Reference 3: 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 4: 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 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 SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 6 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 -Subparagraph (a)(2) Reference 5: 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 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 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 6 Reference 3: 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 4: 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 + Details
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- Definition Represents the amount of adverse loss reserve development on losses occurring in years 2000 and prior. + References No definition available. + 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 reserves ceded as of the date of the OneBeacon acquisition. + References No definition available. + Details
Name: | wtm_LossReservesCededAtAcquisition
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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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- 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 ceded under quota share reinsurance agreement. + References No definition available. + Details
Name: | wtm_PercentageOfQuotaShareReinsuranceAgreement
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Data Type: | num:percentItemType
Balance Type: | na
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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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Data Type: | num:percentItemType
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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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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
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- Definition Represents the retention percentage between $50 million and $100 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
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- Definition Represents the retention percentage between $100 million and $175 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

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- Definition Represents the retention for the second layer of the reinsurance agreement. + References No definition available. + Details
Name: | wtm_PropertyCatastropheReinsuranceProgramSecondRetentionSlabForSecondLayer
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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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

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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 amount of incurred losses paid by the reinsurer through the reporting date. + References No definition available. + Details
Name: | wtm_ReinsuranceReceivablesPaidByReinsurer
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- 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

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- 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

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- Definition Reinsurance recoverables from various third party reinsurers that are guaranteed under the terms of a 100% quota share reinsurance agreement. + References No definition available. + Details
Name: | wtm_ReinsuranceRecoverablesGuaranteedByThirdPartyReinsurer
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant