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v2.4.1.9

Significant Transactions Significant Transactions (Acquisitions) (Details) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended | 3 Months Ended | 0 Months Ended |  |  |  | 3 Months Ended |  |  |  | 2 Months Ended |  |  | 1 Months Ended |  | 12 Months Ended |  |  |  | 2 Months Ended | 3 Months Ended |  | 0 Months Ended | 12 Months Ended |
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Dec. 31, 2014 USD ($) | Sep. 30, 2014 USD ($) | Jun. 30, 2014 USD ($) | Mar. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Sep. 30, 2013 USD ($) | Jun. 30, 2013 USD ($) | Mar. 31, 2013 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2014 Olympus [Member] USD ($) | Oct. 10, 2014 Tranzact [Member] USD ($) | Dec. 31, 2014 Tranzact [Member] USD ($) | Oct. 10, 2014 Tranzact [Member] USD ($) | Dec. 31, 2013 Tranzact [Member] USD ($) | Mar. 31, 2014 QuoteLab [Member] USD ($) | Dec. 31, 2014 QuoteLab [Member] USD ($) | Mar. 14, 2014 QuoteLab [Member] USD ($) | Dec. 31, 2013 QuoteLab [Member] USD ($) | Feb. 19, 2014 WOBI [Member] USD ($) business | Dec. 31, 2014 WOBI [Member] USD ($) | Dec. 31, 2013 WOBI [Member] USD ($) | Jan. 31, 2014 Star & Shield LLC [Member] USD ($) | Dec. 31, 2014 Star & Shield Insurance Exchange [Member] USD ($) | Dec. 31, 2013 HG Global-BAM USD ($) | Dec. 31, 2014 HG Global USD ($) | Dec. 31, 2013 HG Global USD ($) | Feb. 19, 2014 Total WOBI [Member] | Feb. 19, 2014 Israel, New Shekels Preferred stocks WOBI [Member] ILS | Feb. 19, 2014 Israel, New Shekels Total WOBI [Member] ILS | Feb. 19, 2014 United States of America, Dollars Preferred stocks WOBI [Member] USD ($) | Feb. 19, 2014 United States of America, Dollars Total WOBI [Member] USD ($) | Mar. 31, 2013 American Fuji [Member] WM Solutions USD ($) | Mar. 31, 2014 Empire [Member] WM Solutions USD ($) | Feb. 19, 2014 WOBI [Member] USD ($) | Dec. 03, 2012 Runoff Entities [Member] WM Solutions USD ($) | Dec. 31, 2014 Non-controlling Interests Star & Shield Insurance Exchange [Member] USD ($) | Dec. 31, 2015 Scenario, Forecast [Member] QuoteLab [Member] | Oct. 10, 2014 Tangible Assets [Member] Tranzact [Member] USD ($)
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Interest in Subsidiaries and Affiliates |  |  |  |  |  |  |  |  |  |  |  | $ 11.5 us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates / dei_LegalEntityAxis = wtm_OlympusMember | $ 177.7 us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates / dei_LegalEntityAxis = wtm_TranzactMember |  |  |  | $ 28.1 us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates / dei_LegalEntityAxis = wtm_QuoteLabMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets | 10,456.9 us-gaap_Assets |  |  |  | 12,144.3 us-gaap_Assets |  |  |  | 10,456.9 us-gaap_Assets | 12,144.3 us-gaap_Assets |  | 24.6 us-gaap_Assets / dei_LegalEntityAxis = wtm_OlympusMember |  |  | 332.8 us-gaap_Assets / dei_LegalEntityAxis = wtm_TranzactMember |  |  |  | 70.1 us-gaap_Assets / dei_LegalEntityAxis = wtm_QuoteLabMember |  |  |  |  |  | 13.5 us-gaap_Assets / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |  |  |  |  |  |  |  |  | 13.4 us-gaap_Assets / us-gaap_BusinessAcquisitionAxis = wtm_WOBIMember |  |  |  |
Liabilities | 5,917.3 us-gaap_Liabilities |  |  |  | 7,746.9 us-gaap_Liabilities |  |  |  | 5,917.3 us-gaap_Liabilities | 7,746.9 us-gaap_Liabilities |  | 13.1 us-gaap_Liabilities / dei_LegalEntityAxis = wtm_OlympusMember |  |  | 108.7 us-gaap_Liabilities / dei_LegalEntityAxis = wtm_TranzactMember |  |  |  | 10.0 us-gaap_Liabilities / dei_LegalEntityAxis = wtm_QuoteLabMember |  | 0.7 us-gaap_Liabilities / dei_LegalEntityAxis = wtm_WOBIMember |  |  |  | 25.9 us-gaap_Liabilities / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 63.20%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_TranzactMember |  |  |  | 60.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_QuoteLabMember |  |  | 63.30%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_WOBIMember |  |  |  |  |  |  | 54.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_WOBIMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  |  |  |  |  |  |  |  |
Premiums Written, Percentage of Total |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 85.00%wtm_PremiumsWrittenPercentageOfTotal / dei_LegalEntityAxis = wtm_WOBIMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Distributed Earnings |  |  |  |  |  |  |  |  |  |  |  |  | 44.2 us-gaap_ProceedsFromDividendsReceived / dei_LegalEntityAxis = wtm_TranzactMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
White Mountains’s common shareholders’ equity, including portion attributable to NCI | 4,539.6 us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest |  |  |  | 4,397.4 us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest |  |  |  | 4,539.6 us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest | 4,397.4 us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest |  |  |  |  | 281.2 us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest / dei_LegalEntityAxis = wtm_TranzactMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other assets | 356.1 us-gaap_OtherAssets |  |  |  | 437.4 us-gaap_OtherAssets |  |  |  | 356.1 us-gaap_OtherAssets | 437.4 us-gaap_OtherAssets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 41.4 us-gaap_OtherAssets / us-gaap_FairValueByAssetClassAxis = wtm_TangibleAssetsMember / dei_LegalEntityAxis = wtm_TranzactMember
Business Combination, Bargain Purchase, Gain Recognized, Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.9 us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount / us-gaap_BusinessAcquisitionAxis = wtm_AmericanFujiMember / dei_LegalEntityAxis = wtm_WmSolutionsMember | 7.3 us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount / us-gaap_BusinessAcquisitionAxis = wtm_EmpireMember / dei_LegalEntityAxis = wtm_WmSolutionsMember |  | 14.5 us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount / us-gaap_BusinessAcquisitionAxis = wtm_RunoffEntitiesMember / dei_LegalEntityAxis = wtm_WmSolutionsMember |  |  |
Goodwill | 168.9 us-gaap_Goodwill |  |  |  | 0 us-gaap_Goodwill |  |  |  | 168.9 us-gaap_Goodwill | 0 us-gaap_Goodwill | 0 us-gaap_Goodwill |  |  | 145.1 us-gaap_Goodwill / dei_LegalEntityAxis = wtm_TranzactMember | 145.1 us-gaap_Goodwill / dei_LegalEntityAxis = wtm_TranzactMember | 0 us-gaap_Goodwill / dei_LegalEntityAxis = wtm_TranzactMember |  | 18.3 us-gaap_Goodwill / dei_LegalEntityAxis = wtm_QuoteLabMember | 18.3 us-gaap_Goodwill / dei_LegalEntityAxis = wtm_QuoteLabMember | 0 us-gaap_Goodwill / dei_LegalEntityAxis = wtm_QuoteLabMember | 5.5 us-gaap_Goodwill / dei_LegalEntityAxis = wtm_WOBIMember | 5.5 us-gaap_Goodwill / dei_LegalEntityAxis = wtm_WOBIMember | 0 us-gaap_Goodwill / dei_LegalEntityAxis = wtm_WOBIMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible Assets, Net (Excluding Goodwill) | 197.5 us-gaap_IntangibleAssetsNetExcludingGoodwill |  |  |  | 20.7 us-gaap_IntangibleAssetsNetExcludingGoodwill |  |  |  | 197.5 us-gaap_IntangibleAssetsNetExcludingGoodwill | 20.7 us-gaap_IntangibleAssetsNetExcludingGoodwill | 16.9 us-gaap_IntangibleAssetsNetExcludingGoodwill |  |  | 142.8 us-gaap_IntangibleAssetsNetExcludingGoodwill / dei_LegalEntityAxis = wtm_TranzactMember | 146.3 us-gaap_IntangibleAssetsNetExcludingGoodwill / dei_LegalEntityAxis = wtm_TranzactMember | 0 us-gaap_IntangibleAssetsNetExcludingGoodwill / dei_LegalEntityAxis = wtm_TranzactMember |  | 32.5 us-gaap_IntangibleAssetsNetExcludingGoodwill / dei_LegalEntityAxis = wtm_QuoteLabMember | 38.5 us-gaap_IntangibleAssetsNetExcludingGoodwill / dei_LegalEntityAxis = wtm_QuoteLabMember | 0 us-gaap_IntangibleAssetsNetExcludingGoodwill / dei_LegalEntityAxis = wtm_QuoteLabMember |  | 2.5 us-gaap_IntangibleAssetsNetExcludingGoodwill / dei_LegalEntityAxis = wtm_WOBIMember | 0 us-gaap_IntangibleAssetsNetExcludingGoodwill / dei_LegalEntityAxis = wtm_WOBIMember |  |  |  |  |  |  |  |  |  |  |  |  | 2.9 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_BusinessAcquisitionAxis = wtm_WOBIMember |  |  |  |
Percent of gross profit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 62.50%wtm_Percentofgrossprofit / dei_LegalEntityAxis = wtm_QuoteLabMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |
Business Combination, Contingent Consideration, Liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.4 us-gaap_BusinessCombinationContingentConsiderationLiability / dei_LegalEntityAxis = wtm_TranzactMember |  |  |  | 7.9 us-gaap_BusinessCombinationContingentConsiderationLiability / dei_LegalEntityAxis = wtm_QuoteLabMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Businesses and Interest in Affiliates |  |  |  |  |  |  |  |  | 199.2 us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates | (15.9)us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates | 41.3 us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates |  |  |  |  |  |  |  |  |  |  |  |  | 1.8 us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates / dei_LegalEntityAxis = wtm_StarShieldLLCMember |  |  |  |  |  | 31.5 us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates / us-gaap_CurrencyAxis = currency_ILS / us-gaap_InvestmentTypeAxis = us-gaap_PreferredStockMember / dei_LegalEntityAxis = wtm_WOBIMember | 14.4 us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates / us-gaap_CurrencyAxis = currency_ILS / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / dei_LegalEntityAxis = wtm_WOBIMember | 9.0 us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates / us-gaap_CurrencyAxis = currency_USD / us-gaap_InvestmentTypeAxis = us-gaap_PreferredStockMember / dei_LegalEntityAxis = wtm_WOBIMember | 4.1 us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates / us-gaap_CurrencyAxis = currency_USD / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / dei_LegalEntityAxis = wtm_WOBIMember |  |  |  |  |  |  |
Number of lines of business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 wtm_Numberoflinesofbusiness / dei_LegalEntityAxis = wtm_WOBIMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of Surplus Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 17.0 wtm_PurchaseofSurplusNotes / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Non-controlling interest in consolidated subsidiaries | (15.0)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (11.2)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | 4.6 us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (0.5)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | 0.3 us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (1.1)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (11.1)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (0.5)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (22.1)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (12.4)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (14.0)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (12.4)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember |  |
Capital contributions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 594.5 us-gaap_PartnersCapitalAccountContributions / dei_LegalEntityAxis = wtm_HgGlobalBamMember |  |  |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 17.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember |  | $ 503.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_HgGlobalMember | $ 503.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.18) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 12 -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 18 -Article 5 + Details
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- Definition In a business combination in which the amount of net identifiable assets acquired and liabilities assumed exceeds the aggregate consideration transferred or to be transferred (as defined), this element represents the amount of gain recognized by the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (f)(1) -URI http://asc.fasb.org/extlink&oid=7488404&loc=d3e6927-128479 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6571-128477 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6527-128477 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 25 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6396-128476 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6387-128476 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 25 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6393-128476 + Details
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- Definition The cash outflow associated with the acquisition of a controlling interest in another entity or an entity that is related to it but not strictly controlled (for example, an unconsolidated subsidiary, affiliate, joint venture or equity method investment). + 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 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585 + Details
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- Definition The cash outflow associated with the acquisition of or advances to an entity that is related to it but not strictly controlled (for example, an unconsolidated subsidiary, affiliate, and joint venture or equity method investment) or the acquisition of an additional interest in a subsidiary (controlled entity). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585 + Details
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