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Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014 | Mar. 31, 2014 | Dec. 31, 2013
--- | --- | --- | --- | ---
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 5,957.2 us-gaap_InvestmentsFairValueDisclosure | $ 5,991.4 us-gaap_InvestmentsFairValueDisclosure | $ 6,736.7 us-gaap_InvestmentsFairValueDisclosure | $ 6,766.5 us-gaap_InvestmentsFairValueDisclosure
Non-Agency Residential Mortgage [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 85.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_NonAgencyResidentialMortgageMember |  |  |
Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 194.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | 131.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |
Residential Mortgage Backed Securities | Super Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 49.3 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SuperSeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Residential Mortgage Backed Securities | Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 145.0 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Residential Mortgage Backed Securities | Subordinate |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SubordinateMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Prime | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 186.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPrimeFinancingReceivableMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Prime | Residential Mortgage Backed Securities | Super Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 48.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPrimeFinancingReceivableMember / wtm_InvestmentByTrancheLevelsAxis = wtm_SuperSeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Prime | Residential Mortgage Backed Securities | Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 138.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPrimeFinancingReceivableMember / wtm_InvestmentByTrancheLevelsAxis = wtm_SeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Prime | Residential Mortgage Backed Securities | Subordinate |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPrimeFinancingReceivableMember / wtm_InvestmentByTrancheLevelsAxis = wtm_SubordinateMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Non-prime | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 6.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = wtm_ResidentialNonprimeFinancingReceivableMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Non-prime | Residential Mortgage Backed Securities | Super Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = wtm_ResidentialNonprimeFinancingReceivableMember / wtm_InvestmentByTrancheLevelsAxis = wtm_SuperSeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Non-prime | Residential Mortgage Backed Securities | Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 6.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = wtm_ResidentialNonprimeFinancingReceivableMember / wtm_InvestmentByTrancheLevelsAxis = wtm_SeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Non-prime | Residential Mortgage Backed Securities | Subordinate |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = wtm_ResidentialNonprimeFinancingReceivableMember / wtm_InvestmentByTrancheLevelsAxis = wtm_SubordinateMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Sub-prime | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 1.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialSubprimeFinancingReceivableMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Sub-prime | Residential Mortgage Backed Securities | Super Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 1.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialSubprimeFinancingReceivableMember / wtm_InvestmentByTrancheLevelsAxis = wtm_SuperSeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Sub-prime | Residential Mortgage Backed Securities | Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialSubprimeFinancingReceivableMember / wtm_InvestmentByTrancheLevelsAxis = wtm_SeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Sub-prime | Residential Mortgage Backed Securities | Subordinate |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialSubprimeFinancingReceivableMember / wtm_InvestmentByTrancheLevelsAxis = wtm_SubordinateMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |

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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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