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Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Investment securities |  |  |
Fair value investments | $ 5,991.4 us-gaap_InvestmentsFairValueDisclosure | $ 6,766.5 us-gaap_InvestmentsFairValueDisclosure | $ 6,950.7 us-gaap_InvestmentsFairValueDisclosure
Fixed rate CMBS |  |  |
Investment securities |  |  |
Average Basis Points of Subordination | 0.25 wtm_AverageBasisPointsOfSubordination / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFixedRateMember |  |
Fair value investments | 116.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFixedRateMember |  |
Fixed rate CMBS | Super Senior |  |  |
Investment securities |  |  |
Fair value investments | 13.1 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SuperSeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFixedRateMember |  |
Fixed rate CMBS | Senior |  |  |
Investment securities |  |  |
Fair value investments | 61.6 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFixedRateMember |  |
Fixed rate CMBS | Subordinate |  |  |
Investment securities |  |  |
Fair value investments | 41.5 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SubordinateMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFixedRateMember |  |
Floating rate CMBS |  |  |
Investment securities |  |  |
Fair value investments | 120.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFloatingRateMember |  |
Floating rate CMBS | Super Senior |  |  |
Investment securities |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SuperSeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFloatingRateMember |  |
Floating rate CMBS | Senior |  |  |
Investment securities |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFloatingRateMember |  |
Floating rate CMBS | Subordinate |  |  |
Investment securities |  |  |
Fair value investments | 120.7 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SubordinateMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFloatingRateMember |  |
Commercial |  |  |
Investment securities |  |  |
Fair value investments | 236.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 282.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |
Commercial | Super Senior |  |  |
Investment securities |  |  |
Fair value investments | 13.1 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SuperSeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  |
Commercial | Senior |  |  |
Investment securities |  |  |
Fair value investments | 61.6 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  |
Commercial | Subordinate |  |  |
Investment securities |  |  |
Fair value investments | $ 162.2 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SubordinateMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  |
Maximum | Commercial |  |  |
Investment securities |  |  |
Loans and Leases Receivable, Ratio of Nonperforming Loans to All Loans | 1.00%us-gaap_LoansAndLeasesReceivableRatioOfNonperformingLoansToAllLoans / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |

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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph a, f, g -Article 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 + Details
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- Definition Reflects the percentage derived from dividing impaired loans on which interest has ceased to be accrued by total loans. + References No definition available. + Details
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