No definition available.

Name:	srt_MortgageLoansOnRealEstateLineItems
Namespace Prefix:	srt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7)(2))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

+ Details

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

- Definition

Amount of financing receivable originated more than five years prior to current fiscal year.

+ References

+ Details

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

- Definition

Amount of financing receivable originated four years prior to current fiscal year.

+ References

+ Details

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

- Definition

Amount of financing receivable originated in current fiscal year.

+ References

+ Details

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

- Definition

Amount of financing receivable originated in fiscal year prior to current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919232-210447

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922352-210448

+ Details

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

- Definition

Amount of financing receivable originated three years prior to current fiscal year.

+ References

+ Details

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

- Definition

Amount of financing receivable originated two years prior to current fiscal year.

+ References

+ Details

Name:	us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

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(c))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

+ Details

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

- Definition

Amount, before allowance for credit loss, of financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922352-210448

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919230-210447

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 80
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922355-210448

+ Details

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

- Details

Name:	us-gaap_LoanToValueAxis=unm_LoanToValueRatioBelowOrEqualTo65PercentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanToValueAxis=unm_LoanToValueRatioAbove65To75PercentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanToValueAxis=unm_LoanToValueRatioAbove75To85PercentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanToValueAxis=unm_LoanToValueRatioAbove85PercentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	unm_FinancingReceivableOriginationPeriodAxis=unm_OriginatedFiveOrMoreYearsBeforeLastFiscalYearMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	unm_FinancingReceivableOriginationPeriodAxis=unm_OriginatedFourYearsBeforeLastFiscalYearMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	unm_FinancingReceivableOriginationPeriodAxis=unm_OriginatedThreeYearsBeforeLastFiscalYearMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	unm_FinancingReceivableOriginationPeriodAxis=unm_OriginatedTwoYearsBeforeLastFiscalYearMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	unm_FinancingReceivableOriginationPeriodAxis=unm_OriginatedInFiscalYearBeforeLatestFiscalYearMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	unm_FinancingReceivableOriginationPeriodAxis=unm_OriginatedInCurrentFiscalYearMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=unm_CreditRatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=unm_BBBCreditRatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=unm_BBCreditRatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=unm_BCreditRatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Mortgage Loans, Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 2,406.5			$ 2,397.0
Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	1,250.1			1,215.1
Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	1,053.7			1,053.0
Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	45.5			91.4
Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	57.2			37.5
Originated Five or More Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	791.4
Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	392.0
Originated Three Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	332.4
Originated Two Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	390.7
Originated in Fiscal Year before Latest Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	367.5
Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	132.5
Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	794.2
Financing Receivable, Originated Four Years before Latest Fiscal Year	394.6
Financing Receivable, Originated Three Years before Latest Fiscal Year	334.2
Financing Receivable, Originated Two Years before Latest Fiscal Year	393.8
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	370.6
Financing Receivable, Originated in Current Fiscal Year	133.7
Financing Receivable, before Allowance for Credit Loss	2,421.1			2,397.0
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	14.6
Commercial Real Estate \| Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	652.1
Financing Receivable, Originated Four Years before Latest Fiscal Year	245.4
Financing Receivable, Originated Three Years before Latest Fiscal Year	146.8
Financing Receivable, Originated Two Years before Latest Fiscal Year	78.1
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	83.3
Financing Receivable, Originated in Current Fiscal Year	48.5
Financing Receivable, before Allowance for Credit Loss	1,254.2
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	3.9	$ 3.9	$ 2.8
Commercial Real Estate \| Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	65.2
Financing Receivable, Originated Four Years before Latest Fiscal Year	121.8
Financing Receivable, Originated Three Years before Latest Fiscal Year	187.4
Financing Receivable, Originated Two Years before Latest Fiscal Year	315.7
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	287.3
Financing Receivable, Originated in Current Fiscal Year	85.2
Financing Receivable, before Allowance for Credit Loss	1,062.6
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	9.1	7.9	4.6
Commercial Real Estate \| Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	18.9
Financing Receivable, Originated Four Years before Latest Fiscal Year	27.4
Financing Receivable, Originated Three Years before Latest Fiscal Year	0.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0.0
Financing Receivable, Originated in Current Fiscal Year	0.0
Financing Receivable, before Allowance for Credit Loss	46.3
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	0.7	1.0	0.5
Commercial Real Estate \| Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	58.0
Financing Receivable, Originated Four Years before Latest Fiscal Year	0.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	0.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0.0
Financing Receivable, Originated in Current Fiscal Year	0.0
Financing Receivable, before Allowance for Credit Loss	58.0
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	0.9	$ 0.4	$ 0.4
Commercial Real Estate \| Originated Five or More Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	2.8
Commercial Real Estate \| Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	2.6
Commercial Real Estate \| Originated Three Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	1.8
Commercial Real Estate \| Originated Two Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	3.1
Commercial Real Estate \| Originated in Fiscal Year before Latest Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	3.1
Commercial Real Estate \| Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	1.2
A Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	466.3			485.6
A Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	233.0
Financing Receivable, Originated Four Years before Latest Fiscal Year	104.5
Financing Receivable, Originated Three Years before Latest Fiscal Year	52.7
Financing Receivable, Originated Two Years before Latest Fiscal Year	60.3
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	17.1
Financing Receivable, Originated in Current Fiscal Year	0.0
Financing Receivable, before Allowance for Credit Loss	467.6
BBB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	1,894.4			1,911.4
BBB Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	515.2
Financing Receivable, Originated Four Years before Latest Fiscal Year	290.1
Financing Receivable, Originated Three Years before Latest Fiscal Year	281.5
Financing Receivable, Originated Two Years before Latest Fiscal Year	333.5
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	353.5
Financing Receivable, Originated in Current Fiscal Year	133.7
Financing Receivable, before Allowance for Credit Loss	1,907.5
BB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	29.4			0.0
BB Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	29.6
Financing Receivable, Originated Four Years before Latest Fiscal Year	0.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	0.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0.0
Financing Receivable, Originated in Current Fiscal Year	0.0
Financing Receivable, before Allowance for Credit Loss	29.6
B Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	16.4			$ 0.0
B Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	16.4
Financing Receivable, Originated Four Years before Latest Fiscal Year	0.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	0.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0.0
Financing Receivable, Originated in Current Fiscal Year	0.0
Financing Receivable, before Allowance for Credit Loss	$ 16.4