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Transactions with Related Persons (Details) (USD $) | 12 Months Ended |  |  |  |  |  | 12 Months Ended |  |  | 12 Months Ended |  | 12 Months Ended | 1 Months Ended | 12 Months Ended |  |  |  | 0 Months Ended |  | 0 Months Ended | 1 Months Ended
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Dec. 31, 2012 Y | Dec. 31, 2011 | Jan. 07, 2011 | Dec. 31, 2012 OneBeacon | Dec. 31, 2011 OneBeacon | Jan. 07, 2011 Franklin Mutual Advisers | Dec. 31, 2012 Symetra | Dec. 31, 2011 Symetra | Dec. 31, 2010 Symetra | Dec. 31, 2012 Oakum Bay Capital | Dec. 31, 2012 Limited Liability Company | Dec. 31, 2011 Limited Liability Company | Dec. 31, 2010 Limited Liability Company | Dec. 31, 2012 Limited Liability Company OneBeacon | Dec. 31, 2012 White Mountains Advisors LLC Limited Liability Company | Jul. 16, 2012 Oakum Bay Capital | Dec. 31, 2012 Oakum Bay Capital Director | Dec. 31, 2012 Oakum Bay Capital Brother of Director | Dec. 31, 2012 Trimarc Capital Fund | Dec. 31, 2010 Trimarc Capital Fund | Dec. 31, 2006 White Mountains Specialty Underwriting Inc | Dec. 31, 2006 One Beacon Professional Insurance | Dec. 31, 2009 Dowling and Partners Connecticut Fund III Lp Director | Dec. 31, 2012 Dowling and Partners Connecticut Fund III Lp Director | Dec. 31, 2012 Range Of Performance Cycle From 2013 to 2015 [Member] Limited Liability Company | Feb. 28, 2013 Performance Cycle From 2010 to 2012 Limited Liability Company | Dec. 31, 2012 Performance Cycle From 2010 to 2012 Limited Liability Company | Dec. 31, 2010 Performance Cycle From 2010 to 2012 Limited Liability Company | Dec. 31, 2012 Performance Cycle From 2011 to 2013 Limited Liability Company | Dec. 31, 2012 Performance Cycle From 2012 to 2014 Limited Liability Company | Dec. 31, 2012 Maximum Limited Liability Company | Dec. 31, 2012 Minimum Limited Liability Company | Dec. 08, 2012 Treasury Stock | Jul. 12, 2012 Treasury Stock | Dec. 18, 2012 Treasury Stock Beneficial Owner | Jul. 12, 2012 Treasury Stock Beneficial Owner | Dec. 31, 2012 Hedge funds Oakum Bay Capital | Jul. 16, 2012 Preferred Equity Securities Oakum Bay Capital | Dec. 31, 2012 Preferred Equity Securities Oakum Bay Capital
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Advisor fees charged on first $200 million of assets under management (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
First threshold limit of assets under management |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 200,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Advisors fees charged on second $200 million of assets under management (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Second threshold limit of assets under management |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 200,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Advisors fees charged on over $400 million of assets under management (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Third threshold limit of assets under management |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets under management |  |  |  |  |  |  | 27,400,000,000 | 26,300,000,000 |  |  |  |  |  | 533,600,000 | 465,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
ERISA Assets |  |  |  |  |  |  |  |  |  |  |  |  |  | 205,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Asset management total fees earned |  |  |  |  |  |  | 15,900,000 | 15,200,000 | 13,800,000 |  | 6,400,000 | 6,000,000 | 6,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares purchased |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 285,000 | 50,000 |  |  |
Share price |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 519.06 | $ 528.45 |  |  |  |  |
Percentage of target number granted actually received by prospector |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 49.00% |  |  |  |  |  |  |  |  |  |  |  |
Number of performance shares granted |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7,000 |  |  | 9,200 | 8,500 | 7,750 |  |  |  |  |  |  |  |  |
Target number of performance shares granted (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 200.00% | 0.00% |  |  |  |  |  |  |
Payment made with respect to the performance shares granted to the related party |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Performance shares granted at the beginning of each performance cycle |  |  |  |  |  |  |  |  |  |  | 4,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of fees in excess of $500,000 revenue chargeable |  |  |  |  |  |  |  |  |  |  | 6.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of revenue over which 6% of fees chargeable |  |  |  |  |  |  |  |  |  |  | 500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fees from Revenue Sharing Agreement with Related Party |  |  |  |  |  |  |  |  |  |  | 500,000 | 200,000 | 700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Invested in limited partnership investment interests |  |  |  |  |  |  |  |  |  |  | 117,200,000 |  |  | 12,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fees incurred |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incentive fees incurred |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments Owned by Related Party |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 66,000,000 |  |
Total investments | 7,278,100,000 | 8,268,000,000 |  | 2,291,500,000 | 2,707,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  | 9,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 56,811 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incentive fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 88,090 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mangagment and incentive rees received, percent |  |  |  |  |  |  |  |  |  | 40.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchases |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6,000,000 | 4,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,000,000 |
Ownerhsip percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual dividend rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.00%
Annual dividend rate after year four |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.00%
Years Before Dividend Increase | 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Years Until Investments Madatorily Redeemable | 6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related party ownerhship percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 27.50% | 45.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares repurchased |  |  |  |  |  | 140,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase price per share of shares repuchased (in dollars per share) |  |  |  |  |  | $ 342.50 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Related Party Transaction Excess of Market Price Over Purchase Price Per Share of Shares Agreed to be Repurchased |  |  | $ 2.50 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interst in Dowling & Partners Connecticut Fund III, LP (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 33.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowings from Fund III |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7,000,000 | 8,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tax credits generated by related party |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition The amount deducted from an entities' taxes, reflecting a percentage of an amount invested in certain assets spread over the useful life of those assets; includes deferred investment tax credits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -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. + Details
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- Definition Amount paid to managing member or general partner for management of the day-to-day business functions of the limited liability company (LLC) or limited partnership (LP). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864 + Details
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- Definition The cash outflow associated with the purchase of all investments (debt, security, other) during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3213-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 15, 17 -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. + Details
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- Definition Represents the percentage of investment advisory fees levied by the investment adviser on over third specified threshold limit of assets under the management. + References No definition available. + Details
Name: | wtm_InvestmentAdvisoryFeesOnThirdThresholdOfAssetsUnderManagementInBasisPoints
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Investments under management of related party. + References No definition available. + Details
Name: | wtm_InvestmentsOwnedByRelatedParty
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The number of units or percentage investment held in the LLC or LP. + References No definition available. + Details
Name: | wtm_LimitedLiabilityCompanyLlcOrLimitedPartnershipLpOwnershipInterest
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The carrying amount of limited partnership investments an investment adviser manages on behalf of investors. + References No definition available. + Details
Name: | wtm_LimitedPartnershipInvestmentsUnderManagementCarryingAmount
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Mangagment and Incentive Fees Received, Percent + References No definition available. + Details
Name: | wtm_MangagmentAndIncentiveFeesReceivedPercent
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Ownerhsip Percentage of Common Equity + References No definition available. + Details
Name: | wtm_OwnerhsipPercentageOfCommonEquity
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the percentage of fees in excess of the specified amount of revenue is chargeable pursuant to revenue sharing agreement with the related party. + References No definition available. + Details
Name: | wtm_PercentageOfFeesInExcessOfSpecifiedAmountOfRevenueChargeablePursuantToRevenueSharingAgreementWithRelatedParty
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Related Party Ownerhship Percentage + References No definition available. + Details
Name: | wtm_RelatedPartyOwnerhshipPercentage
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the excess of market price per share over the price at which the entity has agreed to repurchase the shares from the transactions with related party. + References No definition available. + Details
Name: | wtm_RelatedPartyTransactionExcessOfMarketPriceOverPurchasePricePerShareOfSharesAgreedToBeRepurchased
Namespace Prefix: | wtm_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition Incentive fees recognized during the period resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party during the period. + References No definition available. + Details
Name: | wtm_RelatedPartyTransactionIncentiveFeesFromTransactionsWithRelatedParty
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Management fees recognized during the period resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party during the period. + References No definition available. + Details
Name: | wtm_RelatedPartyTransactionManagementFeesFromTransactionsWithRelatedParty
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the number of shares repurchased by the entity from transactions with related party during the period. + References No definition available. + Details
Name: | wtm_RelatedPartyTransactionNumberOfSharesAgreedToBeRepurchased
Namespace Prefix: | wtm_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the purchase price per share of shares agreed to be repurchased by the entity from transactions with related party during the period. + References No definition available. + Details
Name: | wtm_RelatedPartyTransactionPurchasePricePerShareOfSharesAgreedToBeRepurchased
Namespace Prefix: | wtm_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the second specified threshold limit of assets under management on which specified percentage of investment advisory fees levied. + References No definition available. + Details
Name: | wtm_SecondSpecifiedThresholdLimitOfAssetsUnderManagement
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Represents the value of performance shares granted at the beginning of each performance cycle to the related party by the entity. + References No definition available. + Details
Name: | wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantedAtBeginningOfPerformanceCycleToRelatedParty
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the payment made with respect to performance shares granted to the related party. + References No definition available. + Details
Name: | wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodPaymentMade
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the percentage of performance shares that are actually awarded as a percentage of the target grant. + References No definition available. + Details
Name: | wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesActuallyAwardedAsPercentageOfTargetAwardsGranted
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the percentage of performance shares that may be awarded as a percentage of the target grant. + References No definition available. + Details
Name: | wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardTargetedPerformanceGoalPercentageOfPayoutForMinimumGrowthTargetPercentage
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents third specified threshold limit of assets under management over which specified percentage of insurance advisory fees levied. + References No definition available. + Details
Name: | wtm_ThirdSpecifiedThresholdLimitOfAssetsUnderManagement
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Years Before Dividend Increase + References No definition available. + Details
Name: | wtm_YearsBeforeDividendIncrease
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Years Until Investments Madatorily Redeemable + References No definition available. + Details
Name: | wtm_YearsUntilInvestmentsMadatorilyRedeemable
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration