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Income Taxes (Net Operating and Capital Loss Carryforwards) (Details) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Net deferred tax assets |  |  |
Total net deferred income tax assets | $ 598,100,000 us-gaap_DeferredTaxAssetsNet | $ 617,600,000 us-gaap_DeferredTaxAssetsNet |
Valuation allowance | 18,100,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance | 33,600,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance | 14,100,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
Alternative minimum tax credit carryforwards, not subject to expiration | 2,000,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax |  |
Tax credit carryforwards | 25,800,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsOther | 23,100,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsOther |
UNITED STATES |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 32,200,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_US |  |
Net deferred tax assets |  |  |
Foreign tax credit carryforwards, subject to expiration | 12,700,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign / us-gaap_IncomeTaxAuthorityAxis = country_US |  |
Tax credit carryforwards | 11,100,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsOther / us-gaap_IncomeTaxAuthorityAxis = country_US |  |
Luxembourg |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 983,400,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_LU |  |
Sirius Re Holdings Inc. |  |  |
Net deferred tax assets |  |  |
Foreign tax credit carryforwards, subject to expiration | 12,100,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign / us-gaap_StatementGeographicalAxis = wtm_SiriusReHoldingsIncMember |  |
Operating and Capital Loss Carryforward |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 2,170,000,000 us-gaap_OperatingLossCarryforwards / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Net deferred tax assets |  |  |
Gross deferred tax asset | 640,800,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Valuation allowance | (265,500,000)us-gaap_OperatingLossCarryforwardsValuationAllowance / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Total net deferred income tax assets | 375,300,000 us-gaap_DeferredTaxAssetsNet / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Operating and Capital Loss Carryforward | UNITED STATES |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 632,300,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_US / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Net deferred tax assets |  |  |
Gross deferred tax asset | 213,400,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_US / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Valuation allowance | (97,200,000)us-gaap_OperatingLossCarryforwardsValuationAllowance / us-gaap_IncomeTaxAuthorityAxis = country_US / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Total net deferred income tax assets | 116,200,000 us-gaap_DeferredTaxAssetsNet / us-gaap_IncomeTaxAuthorityAxis = country_US / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Operating and Capital Loss Carryforward | Luxembourg |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 1,417,500,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_LU / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Net deferred tax assets |  |  |
Gross deferred tax asset | 414,200,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_LU / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Valuation allowance | (166,100,000)us-gaap_OperatingLossCarryforwardsValuationAllowance / us-gaap_IncomeTaxAuthorityAxis = country_LU / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Total net deferred income tax assets | 248,100,000 us-gaap_DeferredTaxAssetsNet / us-gaap_IncomeTaxAuthorityAxis = country_LU / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Operating and Capital Loss Carryforward | SWEDEN |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 106,000,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_SE / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Net deferred tax assets |  |  |
Gross deferred tax asset | 9,800,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_SE / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Valuation allowance | 0 us-gaap_OperatingLossCarryforwardsValuationAllowance / us-gaap_IncomeTaxAuthorityAxis = country_SE / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Total net deferred income tax assets | 9,800,000 us-gaap_DeferredTaxAssetsNet / us-gaap_IncomeTaxAuthorityAxis = country_SE / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Operating and Capital Loss Carryforward | UNITED KINGDOM |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 5,800,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_GB / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Net deferred tax assets |  |  |
Gross deferred tax asset | 1,200,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_GB / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Valuation allowance | 0 us-gaap_OperatingLossCarryforwardsValuationAllowance / us-gaap_IncomeTaxAuthorityAxis = country_GB / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Total net deferred income tax assets | 1,200,000 us-gaap_DeferredTaxAssetsNet / us-gaap_IncomeTaxAuthorityAxis = country_GB / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Operating and Capital Loss Carryforward | Foreign Tax Authority [Member] |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 7,100,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_ForeignCountryMember / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Net deferred tax assets |  |  |
Gross deferred tax asset | 1,900,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_ForeignCountryMember / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Valuation allowance | (1,900,000)us-gaap_OperatingLossCarryforwardsValuationAllowance / us-gaap_IncomeTaxAuthorityAxis = us-gaap_ForeignCountryMember / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Total net deferred income tax assets | 0 us-gaap_DeferredTaxAssetsNet / us-gaap_IncomeTaxAuthorityAxis = us-gaap_ForeignCountryMember / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Operating and Capital Loss Carryforward | NETHERLANDS |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 1,300,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_NL / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Net deferred tax assets |  |  |
Gross deferred tax asset | 300,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_NL / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Valuation allowance | (300,000)us-gaap_OperatingLossCarryforwardsValuationAllowance / us-gaap_IncomeTaxAuthorityAxis = country_NL / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
Total net deferred income tax assets | 0 us-gaap_DeferredTaxAssetsNet / us-gaap_IncomeTaxAuthorityAxis = country_NL / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
No expiration date [Member] | Operating and Capital Loss Carryforward |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 1,536,400,000 us-gaap_OperatingLossCarryforwards / wtm_TaxAssessmentYearAxisAxis = wtm_NoexpirationdateMember / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
No expiration date [Member] | Operating and Capital Loss Carryforward | UNITED STATES |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_US / wtm_TaxAssessmentYearAxisAxis = wtm_NoexpirationdateMember / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
No expiration date [Member] | Operating and Capital Loss Carryforward | Luxembourg |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 1,417,500,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_LU / wtm_TaxAssessmentYearAxisAxis = wtm_NoexpirationdateMember / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
No expiration date [Member] | Operating and Capital Loss Carryforward | SWEDEN |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 106,000,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_SE / wtm_TaxAssessmentYearAxisAxis = wtm_NoexpirationdateMember / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
No expiration date [Member] | Operating and Capital Loss Carryforward | UNITED KINGDOM |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 5,800,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_GB / wtm_TaxAssessmentYearAxisAxis = wtm_NoexpirationdateMember / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
No expiration date [Member] | Operating and Capital Loss Carryforward | Foreign Tax Authority [Member] |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 7,100,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_ForeignCountryMember / wtm_TaxAssessmentYearAxisAxis = wtm_NoexpirationdateMember / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
No expiration date [Member] | Operating and Capital Loss Carryforward | NETHERLANDS |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_NL / wtm_TaxAssessmentYearAxisAxis = wtm_NoexpirationdateMember / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2024-2033 [Member] | Operating and Capital Loss Carryforward |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 621,600,000 us-gaap_OperatingLossCarryforwards / wtm_TaxAssessmentYearAxisAxis = wtm_A20242033Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2024-2033 [Member] | Operating and Capital Loss Carryforward | UNITED STATES |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 621,600,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_US / wtm_TaxAssessmentYearAxisAxis = wtm_A20242033Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2024-2033 [Member] | Operating and Capital Loss Carryforward | Luxembourg |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_LU / wtm_TaxAssessmentYearAxisAxis = wtm_A20242033Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2024-2033 [Member] | Operating and Capital Loss Carryforward | SWEDEN |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_SE / wtm_TaxAssessmentYearAxisAxis = wtm_A20242033Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2024-2033 [Member] | Operating and Capital Loss Carryforward | UNITED KINGDOM |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_GB / wtm_TaxAssessmentYearAxisAxis = wtm_A20242033Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2024-2033 [Member] | Operating and Capital Loss Carryforward | Foreign Tax Authority [Member] |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_ForeignCountryMember / wtm_TaxAssessmentYearAxisAxis = wtm_A20242033Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2024-2033 [Member] | Operating and Capital Loss Carryforward | NETHERLANDS |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_NL / wtm_TaxAssessmentYearAxisAxis = wtm_A20242033Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2019-2013 [Member] | Operating and Capital Loss Carryforward |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 10,400,000 us-gaap_OperatingLossCarryforwards / wtm_TaxAssessmentYearAxisAxis = wtm_A20192013Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2019-2013 [Member] | Operating and Capital Loss Carryforward | UNITED STATES |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 9,600,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_US / wtm_TaxAssessmentYearAxisAxis = wtm_A20192013Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2019-2013 [Member] | Operating and Capital Loss Carryforward | Luxembourg |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_LU / wtm_TaxAssessmentYearAxisAxis = wtm_A20192013Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2019-2013 [Member] | Operating and Capital Loss Carryforward | SWEDEN |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_SE / wtm_TaxAssessmentYearAxisAxis = wtm_A20192013Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2019-2013 [Member] | Operating and Capital Loss Carryforward | UNITED KINGDOM |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_GB / wtm_TaxAssessmentYearAxisAxis = wtm_A20192013Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2019-2013 [Member] | Operating and Capital Loss Carryforward | Foreign Tax Authority [Member] |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_ForeignCountryMember / wtm_TaxAssessmentYearAxisAxis = wtm_A20192013Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2019-2013 [Member] | Operating and Capital Loss Carryforward | NETHERLANDS |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 800,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_NL / wtm_TaxAssessmentYearAxisAxis = wtm_A20192013Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2014-2018 [Member] | Operating and Capital Loss Carryforward |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 1,600,000 us-gaap_OperatingLossCarryforwards / wtm_TaxAssessmentYearAxisAxis = wtm_A20142018Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2014-2018 [Member] | Operating and Capital Loss Carryforward | UNITED STATES |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 1,100,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_US / wtm_TaxAssessmentYearAxisAxis = wtm_A20142018Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2014-2018 [Member] | Operating and Capital Loss Carryforward | Luxembourg |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_LU / wtm_TaxAssessmentYearAxisAxis = wtm_A20142018Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2014-2018 [Member] | Operating and Capital Loss Carryforward | SWEDEN |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_SE / wtm_TaxAssessmentYearAxisAxis = wtm_A20142018Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2014-2018 [Member] | Operating and Capital Loss Carryforward | UNITED KINGDOM |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_GB / wtm_TaxAssessmentYearAxisAxis = wtm_A20142018Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2014-2018 [Member] | Operating and Capital Loss Carryforward | Foreign Tax Authority [Member] |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_ForeignCountryMember / wtm_TaxAssessmentYearAxisAxis = wtm_A20142018Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2014-2018 [Member] | Operating and Capital Loss Carryforward | NETHERLANDS |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 500,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_NL / wtm_TaxAssessmentYearAxisAxis = wtm_A20142018Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2013 [Member] | Operating and Capital Loss Carryforward |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / wtm_TaxAssessmentYearAxisAxis = wtm_A2013Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2013 [Member] | Operating and Capital Loss Carryforward | UNITED STATES |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_US / wtm_TaxAssessmentYearAxisAxis = wtm_A2013Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2013 [Member] | Operating and Capital Loss Carryforward | Luxembourg |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_LU / wtm_TaxAssessmentYearAxisAxis = wtm_A2013Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2013 [Member] | Operating and Capital Loss Carryforward | SWEDEN |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_SE / wtm_TaxAssessmentYearAxisAxis = wtm_A2013Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2013 [Member] | Operating and Capital Loss Carryforward | UNITED KINGDOM |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_GB / wtm_TaxAssessmentYearAxisAxis = wtm_A2013Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2013 [Member] | Operating and Capital Loss Carryforward | Foreign Tax Authority [Member] |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_ForeignCountryMember / wtm_TaxAssessmentYearAxisAxis = wtm_A2013Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |
2013 [Member] | Operating and Capital Loss Carryforward | NETHERLANDS |  |  |
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |
Operating Loss Carryforwards | $ 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = country_NL / wtm_TaxAssessmentYearAxisAxis = wtm_A2013Member / us-gaap_TaxCreditCarryforwardAxis = wtm_OperatingandCapitalLossCarryforwardMember |  |

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- Definition Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b),(c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319 + Details
Name: | us-gaap_DeferredTaxAssetsNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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