Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 948
-SubTopic 310
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479851/948-310-S99-1

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://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(4))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (1)
-Publisher SEC

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (3)
-Publisher SEC

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (c)
-Publisher SEC

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-1

Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7)(2))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-SubTopic 10
-Topic 310
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-11B

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

+ Details

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

- Definition

Amortized cost of financing receivable originated more than five years prior to current fiscal year. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479294/326-20-55-79

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-6

+ Details

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

- Definition

Amortized cost of financing receivable originated four years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost of financing receivable originated in current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost of financing receivable originated in fiscal year prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost of financing receivable originated three years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost of financing receivable originated two years prior to current fiscal year. Excludes net investment in lease.

+ 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://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1)(c))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1

+ Details

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

- Definition

Amortized cost, before allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement and net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 80
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479294/326-20-55-80

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479294/326-20-55-79

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3)(b))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (1)
-Publisher SEC

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (2)
-Publisher SEC

Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Regulation S-K (SK)
-Number 229
-Section 1404
-Paragraph (a)
-Publisher SEC

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-14

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 7A
-SubTopic 10
-Topic 310
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-7A

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-5

+ 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_AACreditRatingMember
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:

Mortgage Loans, Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 2,321.5		$ 2,435.4
Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	1,351.8		1,389.6
Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	762.4		937.2
Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	137.9		75.0
Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	69.4		33.6
Originated Five or More Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	1,353.5		1,104.2
Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	337.0		374.7
Originated Three Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	166.0		343.5
Originated Two Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	349.3		169.3
Originated in Fiscal Year before Latest Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	87.3		355.6
Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	28.4		88.1
Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	1,360.1		1,108.5
Financing Receivable, Originated Four Years before Latest Fiscal Year	338.3		376.5
Financing Receivable, Originated Three Years before Latest Fiscal Year	166.6		344.8
Financing Receivable, Originated Two Years before Latest Fiscal Year	350.3		169.9
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	87.9		356.4
Financing Receivable, Originated in Current Fiscal Year	28.5		88.6
Financing Receivable, before Allowance for Credit Loss	2,331.7		2,444.7
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	10.2		9.3
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	882.6		782.6
Financing Receivable, Originated Four Years before Latest Fiscal Year	199.0		189.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	110.8		193.9
Financing Receivable, Originated Two Years before Latest Fiscal Year	146.2		81.3
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	16.4		128.9
Financing Receivable, Originated in Current Fiscal Year	0.0		16.9
Financing Receivable, before Allowance for Credit Loss	1,355.0		1,392.6
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	3.2	$ 3.3	3.0	$ 2.5	$ 2.3	$ 2.6
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	308.1		230.2
Financing Receivable, Originated Four Years before Latest Fiscal Year	139.3		181.6
Financing Receivable, Originated Three Years before Latest Fiscal Year	47.5		150.9
Financing Receivable, Originated Two Years before Latest Fiscal Year	172.0		80.1
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	71.5		227.5
Financing Receivable, Originated in Current Fiscal Year	28.5		71.7
Financing Receivable, before Allowance for Credit Loss	766.9		942.0
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	4.4	4.8	4.7	5.2	4.8	4.7
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	98.3		67.5
Financing Receivable, Originated Four Years before Latest Fiscal Year	0.0		0.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	8.3		0.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	32.1		8.5
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0.0		0.0
Financing Receivable, Originated in Current Fiscal Year	0.0		0.0
Financing Receivable, before Allowance for Credit Loss	138.7		76.0
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	0.9	0.9	1.1	0.7	0.9	0.7
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	71.1		28.2
Financing Receivable, Originated Four Years before Latest Fiscal Year	0.0		5.9
Financing Receivable, Originated Three Years before Latest Fiscal Year	0.0		0.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0.0		0.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0.0		0.0
Financing Receivable, Originated in Current Fiscal Year	0.0		0.0
Financing Receivable, before Allowance for Credit Loss	71.1		34.1
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	1.7	$ 2.1	0.5	$ 0.3	$ 0.4	$ 0.3
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	6.6		4.3
Commercial Real Estate \| Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	1.3		1.8
Commercial Real Estate \| Originated Three Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	0.6		1.3
Commercial Real Estate \| Originated Two Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	1.0		0.6
Commercial Real Estate \| Originated in Fiscal Year before Latest Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	0.6		0.8
Commercial Real Estate \| Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	0.1		0.5
AA Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	85.9		92.3
AA Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	75.0		53.8
Financing Receivable, Originated Four Years before Latest Fiscal Year	11.0		27.5
Financing Receivable, Originated Three Years before Latest Fiscal Year	0.0		11.1
Financing Receivable, Originated Two Years before Latest Fiscal Year	0.0		0.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0.0		0.0
Financing Receivable, Originated in Current Fiscal Year	0.0		0.0
Financing Receivable, before Allowance for Credit Loss	86.0		92.4
A Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	885.4		843.9
A Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	585.1		485.0
Financing Receivable, Originated Four Years before Latest Fiscal Year	101.0		123.3
Financing Receivable, Originated Three Years before Latest Fiscal Year	71.3		96.8
Financing Receivable, Originated Two Years before Latest Fiscal Year	90.9		35.9
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	23.4		80.6
Financing Receivable, Originated in Current Fiscal Year	15.5		24.0
Financing Receivable, before Allowance for Credit Loss	887.2		845.6
BBB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	1,309.1		1,458.0
BBB Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	657.6		534.0
Financing Receivable, Originated Four Years before Latest Fiscal Year	226.3		219.8
Financing Receivable, Originated Three Years before Latest Fiscal Year	95.3		236.9
Financing Receivable, Originated Two Years before Latest Fiscal Year	259.4		134.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	64.5		275.8
Financing Receivable, Originated in Current Fiscal Year	13.0		64.6
Financing Receivable, before Allowance for Credit Loss	1,316.1		1,465.1
BB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	41.1		41.2
BB Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	42.4		35.7
Financing Receivable, Originated Four Years before Latest Fiscal Year	0.0		5.9
Financing Receivable, Originated Three Years before Latest Fiscal Year	0.0		0.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0.0		0.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0.0		0.0
Financing Receivable, Originated in Current Fiscal Year	0.0		0.0
Financing Receivable, before Allowance for Credit Loss	$ 42.4		$ 41.6