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Third Party Reinsurance (One Beacon) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
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Mar. 31, 2015 | Dec. 31, 2014
Reinsurance Retention Policy [Line Items] |  |
Reinsurance recoverable on paid losses | 29.8 us-gaap_ReinsuranceRecoverablesOnPaidLosses | $ 23.6 us-gaap_ReinsuranceRecoverablesOnPaidLosses
Reinsurance recoverable on unpaid losses | 436.9 us-gaap_ReinsuranceRecoverablesOnUnpaidLosses | 483.9 us-gaap_ReinsuranceRecoverablesOnUnpaidLosses
OneBeacon |  |
Reinsurance Retention Policy [Line Items] |  |
Reinsurance recoverable on paid losses | 18.2 us-gaap_ReinsuranceRecoverablesOnPaidLosses / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 12.2 us-gaap_ReinsuranceRecoverablesOnPaidLosses / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Reinsurance recoverable on unpaid losses | 141.6 us-gaap_ReinsuranceRecoverablesOnUnpaidLosses / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | $ 161.6 us-gaap_ReinsuranceRecoverablesOnUnpaidLosses / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Crop insurance [Member] | OneBeacon |  |
Reinsurance Retention Policy [Line Items] |  |
Percentage of par value of policy reinsured | 52.00%us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CropinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage | 98.00%wtm_ReinsuranceRetentionPolicyLossRatioReinsuredriskPercentage / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CropinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |
Percentage of Quota Share Reinsurance Agreement | 30.00%wtm_PercentageOfQuotaShareReinsuranceAgreement / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CropinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |
Crop-hail insurance [Member] | OneBeacon |  |
Reinsurance Retention Policy [Line Items] |  |
Percentage of par value of policy reinsured | 80.00%us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CrophailinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage | 100.00%wtm_ReinsuranceRetentionPolicyLossRatioReinsuredriskPercentage / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CrophailinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |

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- Definition Amount after valuation allowance of reinsurance recoverables on paid losses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 + Details
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- Definition Amount after valuation allowance of reinsurance recoverables for losses reported to the ceding insurer but not yet paid, amounts expected of incurred losses and settlement expenses, which have not yet been reported to the ceding insurer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 + Details
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- Definition Percentage of risk reinsured on insurance products per entity policy. + References No definition available. + Details
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- Definition Represents the percentage ceded under quota share reinsurance agreement. + References No definition available. + Details
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- Definition Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage + References No definition available. + Details
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