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SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014
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Summary of investments other than investments in related parties |
Cost | $ 6,499.7 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost
Carrying Value | 6,863.1 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount
Fair Value | 6,863.1 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue
Fixed maturity investments |
Summary of investments other than investments in related parties |
Cost | 4,653.3 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_FixedMaturitiesMember
Carrying Value | 4,784.3 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_FixedMaturitiesMember
Fair Value | 4,784.3 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_FixedMaturitiesMember
US Government and agency obligations |
Summary of investments other than investments in related parties |
Cost | 184.7 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_USTreasuryAndGovernmentMember
Carrying Value | 188.1 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_USTreasuryAndGovernmentMember
Fair Value | 188.1 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_USTreasuryAndGovernmentMember
Debt securities issued by corporations |
Summary of investments other than investments in related parties |
Cost | 2,221.3 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_CorporateDebtSecuritiesMember
Carrying Value | 2,311.2 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_CorporateDebtSecuritiesMember
Fair Value | 2,311.2 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_CorporateDebtSecuritiesMember
Municipal obligations |
Summary of investments other than investments in related parties |
Cost | 1,811.1 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_MunicipalBondsMember
Carrying Value | 1,840.9 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_MunicipalBondsMember
Fair Value | 1,840.9 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_MunicipalBondsMember
Mortgage-backed and asset-backed securities |
Summary of investments other than investments in related parties |
Cost | 82.0 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_AssetBackedSecuritiesMember
Carrying Value | 83.2 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_AssetBackedSecuritiesMember
Fair Value | 83.2 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_AssetBackedSecuritiesMember
Foreign government, agency and provincial obligations |
Summary of investments other than investments in related parties |
Cost | 274.6 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Carrying Value | 275.1 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Fair Value | 275.1 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Redeemable preferred stocks |
Summary of investments other than investments in related parties |
Cost | 79.6 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_PreferredStockMember
Carrying Value | 85.8 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_PreferredStockMember
Fair Value | 85.8 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_PreferredStockMember
Short-term investments |
Summary of investments other than investments in related parties |
Cost | 871.7 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ShortTermInvestmentsMember
Carrying Value | 871.7 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ShortTermInvestmentsMember
Fair Value | 871.7 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ShortTermInvestmentsMember
Common equity securities |
Summary of investments other than investments in related parties |
Cost | 633.6 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Carrying Value | 801.6 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Fair Value | 801.6 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Banks, trust and insurance companies |
Summary of investments other than investments in related parties |
Cost | 167.0 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_BanksTrustAndInsuranceEquitiesMember
Carrying Value | 233.9 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_BanksTrustAndInsuranceEquitiesMember
Fair Value | 233.9 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_BanksTrustAndInsuranceEquitiesMember
Public utilities |
Summary of investments other than investments in related parties |
Cost | 7.9 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_PublicUtilityEquitiesMember
Carrying Value | 9.5 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_PublicUtilityEquitiesMember
Fair Value | 9.5 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_PublicUtilityEquitiesMember
Industrial, miscellaneous and other |
Summary of investments other than investments in related parties |
Cost | 458.7 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_IndustrialMiscellaneousAndAllOthersMember
Carrying Value | 558.2 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_IndustrialMiscellaneousAndAllOthersMember
Fair Value | 558.2 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_IndustrialMiscellaneousAndAllOthersMember
Convertible fixed maturity and preferred investments |
Summary of investments other than investments in related parties |
Cost | 19.1 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ConvertibleDebtSecuritiesMember
Carrying Value | 20.5 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ConvertibleDebtSecuritiesMember
Fair Value | 20.5 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ConvertibleDebtSecuritiesMember
Other long-term investments |
Summary of investments other than investments in related parties |
Cost | 322.0 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_OtherLongTermInvestmentsMember
Carrying Value | 385.0 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_OtherLongTermInvestmentsMember
Fair Value | 385.0 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_OtherLongTermInvestmentsMember
Other long-term investments |
Summary of investments other than investments in related parties |
Equity Method Investment, Carrying Value | 22.5 wtm_EquityMethodInvestmentCarryingValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Carrying value of investment accounted for using the equity method | $ 23.2 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember

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