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Income Taxes (Tax Examinations) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Mar. 31, 2014
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Income Tax Examination [Line Items] |  |
Unrecognized tax benefit recognition including interest and penalties, if recognized, would effect income tax expense | $ 27.1 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate | $ 5.0 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Tax Assessment Year 2005 to 2006 [Member] | IRS |  |
Income Tax Examination [Line Items] |  |
Tax Examination | 3.3 us-gaap_IncomeTaxExaminationIncreaseDecreaseInLiabilityFromPriorYear / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember / wtm_TaxAssessmentYearAxisAxis = wtm_TaxAssessmentYear2005To2006Member |
Tax Assessment Year 2007 to 2009 [Member] | IRS |  |
Income Tax Examination [Line Items] |  |
Tax Examination | 72.9 us-gaap_IncomeTaxExaminationIncreaseDecreaseInLiabilityFromPriorYear / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember / wtm_TaxAssessmentYearAxisAxis = wtm_TaxAssessmentYear2007To2009Member |
Income Tax Examination, Increase (Decrease) in Liability from Prior Year, Dissallowed | $ 60.2 wtm_IncomeTaxExaminationIncreaseDecreaseinLiabilityfromPriorYearDissallowed / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember / wtm_TaxAssessmentYearAxisAxis = wtm_TaxAssessmentYear2007To2009Member |

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- Definition The amount the liability recorded for the income tax examination increased or decreased in the current year/accounting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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- Definition The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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