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Third Party Reinsurance (Recoverable Amounts by Reinsurer) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Reinsurance Retention Policy [Line Items] |  |
Reinsurance recoverable on paid and unpaid losses | $ 507.5 us-gaap_ReinsuranceRecoverablesGross | $ 453.5 us-gaap_ReinsuranceRecoverablesGross
Sirius Group |  |
Reinsurance Retention Policy [Line Items] |  |
Reinsurance recoverable on paid and unpaid losses | 333.6 us-gaap_ReinsuranceRecoverablesGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 363.6 us-gaap_ReinsuranceRecoverablesGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember
Reinsurance Recoverables Allowance | 30.2 us-gaap_ReinsuranceRecoverablesAllowance / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance Recoverables | 303.4 us-gaap_ReinsuranceRecoverables / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance recoverables, percentage of total | 100.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
AA | Sirius Group |  |
Reinsurance Retention Policy [Line Items] |  |
Reinsurance recoverable on paid and unpaid losses | 114.6 us-gaap_ReinsuranceRecoverablesGross / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance Recoverables Allowance | 1.6 us-gaap_ReinsuranceRecoverablesAllowance / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance Recoverables | 113.0 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance recoverables, percentage of total | 34.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
A | Sirius Group |  |
Reinsurance Retention Policy [Line Items] |  |
Reinsurance recoverable on paid and unpaid losses | 138.8 us-gaap_ReinsuranceRecoverablesGross / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance Recoverables Allowance | 7.1 us-gaap_ReinsuranceRecoverablesAllowance / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance Recoverables | 131.7 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance recoverables, percentage of total | 42.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
BBB plus | Sirius Group |  |
Reinsurance Retention Policy [Line Items] |  |
Reinsurance recoverable on paid and unpaid losses | 8.9 us-gaap_ReinsuranceRecoverablesGross / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance Recoverables Allowance | 0 us-gaap_ReinsuranceRecoverablesAllowance / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance Recoverables | 8.9 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance recoverables, percentage of total | 3.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
BBB | Sirius Group |  |
Reinsurance Retention Policy [Line Items] |  |
Reinsurance recoverable on paid and unpaid losses | 9.6 us-gaap_ReinsuranceRecoverablesGross / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBRatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance Recoverables Allowance | 0 us-gaap_ReinsuranceRecoverablesAllowance / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBRatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance Recoverables | 9.6 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBRatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance recoverables, percentage of total | 3.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBRatingMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Not rated | Sirius Group |  |
Reinsurance Retention Policy [Line Items] |  |
Reinsurance recoverable on paid and unpaid losses | 61.7 us-gaap_ReinsuranceRecoverablesGross / us-gaap_CreditRatingStandardPoorsAxis = wtm_NotRatedMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance Recoverables Allowance | 21.5 us-gaap_ReinsuranceRecoverablesAllowance / us-gaap_CreditRatingStandardPoorsAxis = wtm_NotRatedMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance Recoverables | $ 40.2 us-gaap_ReinsuranceRecoverables / us-gaap_CreditRatingStandardPoorsAxis = wtm_NotRatedMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Reinsurance recoverables, percentage of total | 18.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal / us-gaap_CreditRatingStandardPoorsAxis = wtm_NotRatedMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |

X
- Definition Amount after valuation allowance recoverable including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387 Reference 3: 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.(a),6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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X
- Definition Amount of valuation allowance for reinsurance recoverables. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References No definition available. + Details
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X
- Definition Amount before valuation allowance recoverable under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References No definition available. + Details
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- Definition Percentage of total reinsurance recoverables. + References No definition available. + Details
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