Fair Values of Financial Instruments Carrying Amounts and Estimated Fair value of Financial Instruments Not Measured at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Mortgage Loans, Carrying Amount	$ 2,499.4	$ 2,432.1
Policy Loans	3,691.4	3,683.9
Other Long-term Investments	1,124.5	960.2
Long-term Debt	3,441.4	3,345.7
Unfunded Commitments	0.7
Liabilities, Fair Value Disclosure	54.0	99.5
Fair Value, Nonrecurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Mortgage Loans, Fair Value Disclosure	2,600.5	2,641.8
Mortgage Loans, Carrying Amount	2,499.4	2,432.1
Policy Loans, Fair Value Disclosure	3,835.2	3,850.8
Policy Loans	3,691.4	3,683.9
Miscellaneous Long-Term Investments, Fair Value Disclosure	38.1	57.5
Other Long-term Investments	38.1	57.5
Financial Instruments, Financial Assets Fair Value Disclosure	6,473.8	6,550.1
Total Financial Instrument Assets Not Carried at Fair Value	6,228.9	6,173.5
Long-term Debt, Fair Value Disclosure	3,906.3	3,887.4
Long-term Debt	3,441.4	3,345.7
Federal Home Loan Bank Funding Agreements, Fair Value Disclosure	261.0	312.2
Federal Home Loan Bank Funding Agreements	261.0	312.2
Unfunded Commitments	0.7	0.9
Liabilities, Fair Value Disclosure	4,168.0	4,200.5
Total Financial Instrument Liabilities Not Carried at Fair Value	3,703.1	3,658.8
Fair Value, Inputs, Level 1 \| Fair Value, Nonrecurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Mortgage Loans, Fair Value Disclosure	0.0	0.0
Policy Loans, Fair Value Disclosure	0.0	0.0
Miscellaneous Long-Term Investments, Fair Value Disclosure	0.0	0.0
Financial Instruments, Financial Assets Fair Value Disclosure	0.0	0.0
Long-term Debt, Fair Value Disclosure	2,929.5	2,393.1
Federal Home Loan Bank Funding Agreements, Fair Value Disclosure	0.0	0.0
Unfunded Commitments	0.0	0.0
Liabilities, Fair Value Disclosure	2,929.5	2,393.1
Fair Value, Inputs, Level 2 \| Fair Value, Nonrecurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Mortgage Loans, Fair Value Disclosure	2,600.5	2,641.8
Policy Loans, Fair Value Disclosure	0.0	0.0
Miscellaneous Long-Term Investments, Fair Value Disclosure	23.8	28.2
Financial Instruments, Financial Assets Fair Value Disclosure	2,624.3	2,670.0
Long-term Debt, Fair Value Disclosure	976.8	1,494.3
Federal Home Loan Bank Funding Agreements, Fair Value Disclosure	261.0	312.2
Unfunded Commitments	0.7	0.9
Liabilities, Fair Value Disclosure	1,238.5	1,807.4
Fair Value, Inputs, Level 3 \| Fair Value, Nonrecurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Mortgage Loans, Fair Value Disclosure	0.0	0.0
Policy Loans, Fair Value Disclosure	3,835.2	3,850.8
Miscellaneous Long-Term Investments, Fair Value Disclosure	14.3	29.3
Financial Instruments, Financial Assets Fair Value Disclosure	3,849.5	3,880.1
Long-term Debt, Fair Value Disclosure	0.0	0.0
Federal Home Loan Bank Funding Agreements, Fair Value Disclosure	0.0	0.0
Unfunded Commitments	0.0	0.0
Liabilities, Fair Value Disclosure	$ 0.0	$ 0.0

Financial Instruments, Financial Assets Fair Value Disclosure, Carrying Value

No definition available.

Name:	unm_FinancialInstrumentsFinancialAssetsFairValueDisclosureCarryingValue
Namespace Prefix:	unm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financial Instruments, Financial Assets Fair Value Disclosure, Total

+ References

No definition available.

+ Details

Name:	unm_FinancialInstrumentsFinancialAssetsFairValueDisclosureTotal
Namespace Prefix:	unm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Miscellaneous Long-Term Investments

+ References

No definition available.

+ Details

Name:	unm_MiscellaneousLongTermInvestments
Namespace Prefix:	unm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Total Financial Instrument Liabilities Not Carried at Fair Value

+ References

No definition available.

+ Details

Name:	unm_TotalFinancialInstrumentLiabilitiesNotCarriedatFairValue
Namespace Prefix:	unm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of Federal Home Loan Bank (FHLBank) advances to member financial institutions.

+ References

No definition available.

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvances
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of borrowing from Federal Home Loan Bank (FHLBank).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13(3)(a))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_FederalHomeLoanBankBorrowingsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value of financial and nonfinancial obligations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

+ Details

Name:	us-gaap_LiabilitiesFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount before allowance of loans made to policy and contract holders using their life insurance contracts' cash value or separate accounts as collateral. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums over and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(e))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(a)(7))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_LoansInsurancePolicy
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

+ Details

Name:	us-gaap_LoansReceivableFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of long-term debt and lease obligation, classified as noncurrent.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebtAndCapitalLeaseObligations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.

+ References

+ Details

Name:	us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of mortgage loans held-for-sale.

+ References

No definition available.

+ Details

Name:	us-gaap_MortgagesHeldForSaleFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of notes payable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20,22)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_NotesPayableFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term investments classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(f))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_OtherLongTermInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of commitment or contingent commitments to qualified affordable housing project investments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 740
-Section 50
-Paragraph 2
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=116633289&loc=SL49122993-202985

+ Details

Name:	us-gaap_QualifiedAffordableHousingProjectInvestmentsCommitment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: