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Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Loss Contingencies [Line Items] |  |  |
Rent expense | $ 19.4 us-gaap_OperatingLeasesRentExpenseNet | $ 20.2 us-gaap_OperatingLeasesRentExpenseNet | $ 18.8 us-gaap_OperatingLeasesRentExpenseNet
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract] |  |  |
Capital lease payments due next year | 16.5 us-gaap_CapitalLeasesFutureMinimumPaymentsDueCurrent |  |
Capital lease payments due in two years | 15.2 us-gaap_CapitalLeasesFutureMinimumPaymentsDueInTwoYears |  |
Capital lease payments due in three years | 13.6 us-gaap_CapitalLeasesFutureMinimumPaymentsDueInThreeYears |  |
Capital Lease payments due in four years and thereafter | 54.7 us-gaap_CapitalLeasesFutureMinimumPaymentsDueInFourYears |  |
Commitments to fund other-long term investments | 81.7 wtm_CommitmentsOtherLongTermInvestments |  |
Reserve for probable liability from mandatory shared market mechanisms | 11.9 us-gaap_LossContingencyUndiscountedAmountOfInsuranceRelatedAssessmentLiability |  |
OB Services |  |  |
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract] |  |  |
Capital lease payments due next year | 5.3 us-gaap_CapitalLeasesFutureMinimumPaymentsDueCurrent / dei_LegalEntityAxis = wtm_OneBeaconServicesLlcMember |  |
Capital lease payments due in two years | 5.3 us-gaap_CapitalLeasesFutureMinimumPaymentsDueInTwoYears / dei_LegalEntityAxis = wtm_OneBeaconServicesLlcMember |  |
Capital lease payments due in three years | 1.8 us-gaap_CapitalLeasesFutureMinimumPaymentsDueInThreeYears / dei_LegalEntityAxis = wtm_OneBeaconServicesLlcMember |  |
Cash proceeds received a result of entering into the sale-leaseback transactions | 23.1 us-gaap_SaleLeasebackTransactionNetProceeds / dei_LegalEntityAxis = wtm_OneBeaconServicesLlcMember |  |
Capital lease obligation | 7.1 us-gaap_CapitalLeaseObligations / dei_LegalEntityAxis = wtm_OneBeaconServicesLlcMember | 12.5 us-gaap_CapitalLeaseObligations / dei_LegalEntityAxis = wtm_OneBeaconServicesLlcMember |
Capital lease asset | $ 7.0 us-gaap_CapitalLeasedAssetsGross / dei_LegalEntityAxis = wtm_OneBeaconServicesLlcMember | $ 10.9 us-gaap_CapitalLeasedAssetsGross / dei_LegalEntityAxis = wtm_OneBeaconServicesLlcMember |
Furniture and equipment | OB Services |  |  |
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract] |  |  |
Capital lease term (in years) | 5 years |  |
Capitalized software | OB Services |  |  |
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract] |  |  |
Capital lease term (in years) | 4 years |  |

X
- Definition Amount before accumulated depreciation of leased physical assets used in the normal conduct of business to produce goods and services. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6455314&loc=d3e45014-112735 + Details
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- Definition Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737 + Details
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- Definition Amount of minimum lease payments maturing in the next fiscal year following the latest fiscal year for capital leases. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737 + Details
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- Definition Amount of minimum lease payments maturing in the forth fiscal year following the latest fiscal year for capital leases. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737 + Details
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- Definition Amount of minimum lease payments maturing in the third fiscal year following the latest fiscal year for capital leases. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737 + Details
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- Definition Amount of minimum lease payments maturing in the second fiscal year following the latest fiscal year for capital leases. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737 + Details
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- Definition The liability as of the balance sheet date representing required funding mandated by statute or regulatory authority that is related directly or indirectly to underwriting activities, including pools for self-insurance and excluding premium taxes and income taxes. The funds derived from the assessments generally enable a government guarantor to pay insurance claims in the event that the underwriting insurer is insolvent or otherwise unable to meet its payment obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 405 -SubTopic 30 -Section 15 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=9154593&loc=d3e23169-109297 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 405 -SubTopic 30 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6392405&loc=d3e24119-109301 + Details
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- Definition Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41499-112717 + Details
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- Definition The gross proceeds received from the asset(s) sold in connection with the transaction involving the sale of property to another party and the lease of the property back to the seller less the costs incurred in connection with the transaction, such as closing and deferred financing costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 40 -Section 55 -Paragraph 51 -URI http://asc.fasb.org/extlink&oid=28365400&loc=d3e51840-112757 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 40 -Section 55 -Paragraph 52 -URI http://asc.fasb.org/extlink&oid=28365400&loc=d3e51843-112757 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 40 -Section 55 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=28365400&loc=d3e51831-112757 + Details
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