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Investment Securities (Other long-term investments) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014 | Mar. 31, 2014 | Dec. 31, 2013
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other Investments | $ 402.6 us-gaap_OtherInvestments | $ 407.0 us-gaap_OtherInvestments |  |
Partnership investments [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other Investments | 3.8 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_PartnershipinvestmentsMember | 5.2 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_PartnershipinvestmentsMember |  |
Hedge and private equity funds included in other long-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other Investments | 242.5 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_HedgeFundsAndPrivateEquityFundsMember | 242.9 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_HedgeFundsAndPrivateEquityFundsMember |  |
Surplus Note |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other Investments | 68.0 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_SurplusNoteMember | 65.1 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_SurplusNoteMember |  |
Limited Liability Companies and Private Equity Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other Investments | 66.6 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_LimitedliabilitycompaniesandprivateequitysecuritiesMember | 69.7 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_LimitedliabilitycompaniesandprivateequitysecuritiesMember |  |
Affordable Housing Development Fund [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other Investments | 16.2 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_AffordableHousingDevelopmentFundMember | 16.8 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_AffordableHousingDevelopmentFundMember | 18.5 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_AffordableHousingDevelopmentFundMember | 19.1 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = wtm_AffordableHousingDevelopmentFundMember
Other |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Other Investments | $ 5.5 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = us-gaap_OtherDebtSecuritiesMember | $ 7.3 us-gaap_OtherInvestments / us-gaap_DebtSecurityAxis = us-gaap_OtherDebtSecuritiesMember |  |

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- Definition Other investments not otherwise specified in the taxonomy. + 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 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 12 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph g -Article 7 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 210.7-03.1(f,g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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