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Schedule I Summary of Investments (Detail) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014
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Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | $ 45,491.4 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost | [1]
Amount Shown on the Balance Sheet | 51,794.3 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount |
United States Government and Government Agencies and Authorities |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | 983.5 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_USTreasuryAndGovernmentMember | [1]
Fair Value of Investments | 1,238.5 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_USTreasuryAndGovernmentMember |
Amount Shown on the Balance Sheet | 1,238.5 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_USTreasuryAndGovernmentMember |
States, Municipalities, and Political Subdivisions |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | 1,745.0 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | [1]
Fair Value of Investments | 2,121.5 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Amount Shown on the Balance Sheet | 2,121.5 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Foreign Government Debt |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | 1,101.1 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ForeignGovernmentDebtMember | [1]
Fair Value of Investments | 1,307.4 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ForeignGovernmentDebtMember |
Amount Shown on the Balance Sheet | 1,307.4 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ForeignGovernmentDebtMember |
Public Utilities |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | 7,046.1 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_PublicUtilityBondsMember | [1]
Fair Value of Investments | 8,550.6 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_PublicUtilityBondsMember |
Amount Shown on the Balance Sheet | 8,550.6 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_PublicUtilityBondsMember |
Mortgage/Asset-backed Securities |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | 2,224.9 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_AssetBackedSecuritiesMember | [1]
Fair Value of Investments | 2,431.8 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_AssetBackedSecuritiesMember |
Amount Shown on the Balance Sheet | 2,431.8 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_AssetBackedSecuritiesMember |
All Other Corporate Bonds |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | 25,658.8 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_AllOtherCorporateBondsMember | [1]
Fair Value of Investments | 29,365.2 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_AllOtherCorporateBondsMember |
Amount Shown on the Balance Sheet | 29,365.2 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_AllOtherCorporateBondsMember |
Redeemable Preferred Stock |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | 44.0 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_MandatorilyRedeemablePreferredStockMember | [1]
Fair Value of Investments | 49.9 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_MandatorilyRedeemablePreferredStockMember |
Amount Shown on the Balance Sheet | 49.9 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_MandatorilyRedeemablePreferredStockMember |
Fixed Maturity Securities |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | 38,803.4 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_FixedMaturitiesMember | [1]
Fair Value of Investments | 45,064.9 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_FixedMaturitiesMember |
Amount Shown on the Balance Sheet | 45,064.9 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_FixedMaturitiesMember |
Mortgage Loans |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | 1,856.6 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_MortgageLoansOnRealEstateMember | [1]
Amount Shown on the Balance Sheet | 1,856.6 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_MortgageLoansOnRealEstateMember |
Policy Loans |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | 3,306.6 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_PolicyLoansMember | [1]
Amount Shown on the Balance Sheet | 3,306.6 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_PolicyLoansMember |
Derivatives |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | 0 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_DerivativeMember | [1]
Amount Shown on the Balance Sheet | 28.0 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_DerivativeMember | [2]
Equity Securities |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | 7.9 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesInvestmentSummaryMember | [1]
Amount Shown on the Balance Sheet | 12.5 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesInvestmentSummaryMember |
Miscellaneous Long-term Investments |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | 542.6 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = unm_MiscellaneousLongtermInvestmentsMember | [1]
Amount Shown on the Balance Sheet | 551.4 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = unm_MiscellaneousLongtermInvestmentsMember | [3]
Short-term Investments |  |
Summary of Investments, Other than Investments in Related Parties, Reportable Data |  |
Cost or Amortized Cost of Investments | 974.3 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ShortTermInvestmentsMember | [1]
Amount Shown on the Balance Sheet | $ 974.3 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ShortTermInvestmentsMember |
[1] | The amortized cost for fixed maturity securities and mortgage loans represents original cost reduced by repayments, write-downs from other-than-temporary declines in fair value, amortization of premiums, and/or accretion of discounts.
[2] | Derivatives are carried at fair value.
[3] | The difference between amortized cost and carrying value primarily results from changes in the partnership owner's equity since acquisition.

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