Notional Amount Of Derivatives, Additions

No definition available.

Name:	unm_NotionalAmountOfDerivativesAdditions
Namespace Prefix:	unm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Notional Amount Of Derivatives, Terminations

+ References

No definition available.

+ Details

Name:	unm_NotionalAmountOfDerivativesTerminations
Namespace Prefix:	unm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121549185&loc=d3e80748-113994

+ Details

Name:	us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The premium (cost) of a hedge, expensed during the period.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeCostOfHedge
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=121570589&loc=SL51823488-111719

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624163-113959

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

+ Details

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

- Definition

Amount of decrease in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5618551-113959

+ Details

Name:	us-gaap_DerivativeLossOnDerivative
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624258-113959

+ Details

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

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

This item represents disclosure of all significant concentrations of credit risk or market risk arising from the subject financial instrument (as defined), whether from an individual counterparty or groups of counterparties.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 15
-Paragraph 83
-URI http://asc.fasb.org/extlink&oid=121636179&loc=d3e34841-113949

+ Details

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

- Definition

Net gain (loss) relating to components of the gain (loss) on the fair value hedging instrument excluded from the assessment of fair value hedge effectiveness. Recognized in earnings.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 25
-Section 50
-Paragraph 1
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=121577467&loc=d3e76258-113986

+ Details

Name:	us-gaap_GainLossFromComponentsExcludedFromAssessmentOfFairValueHedgeEffectivenessNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of net gain (loss) recognized in earnings when a hedged firm commitment no longer qualifies as a fair value hedge.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 25
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121577467&loc=d3e76258-113986

+ Details

Name:	us-gaap_GainLossFromHedgedFirmCommitmentNotQualifyingAsFairValueHedgeNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_FairValueHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CreditRiskContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditDerivativesByContractTypeAxis=us-gaap_CreditDefaultSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	unm_CreditExposureAxis=unm_CreditExposureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	unm_InterestRateDerivativesContractTypeAxis=unm_DerivativeFinancialInstrumentsReceiveFixedPayFixedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	unm_InterestRateDerivativesContractTypeAxis=unm_DerivativeFinancialInstrumentsReceiveFixedPayVariableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Derivative
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	$ 34.0		$ 34.0			$ 34.6
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months			72.8
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness	(10.2)	$ (0.6)	14.1	$ 0.9
Discontinued Hedge Accounting Due to Instrument No Longer Qualifying as Fair Value Hedge			0.0	0.0
Derivative, Notional Amount	721.6	930.8	721.6	930.8	$ 931.4	881.4	$ 819.7	$ 799.2
Additions	43.1	380.9	95.1	504.0
Notional Amount Of Derivatives Terminations	253.3	269.5	254.6	371.6
Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	9.9		9.9			12.2
Not Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	77.8		77.8			45.2
Fair Value Hedging \| Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability						5.6
Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability						0.6
Credit Risk Contract \| Not Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	0.0		0.0			0.0
Credit Risk Contract \| Credit Default Swaps
Derivative
Derivative, Notional Amount	11.1	10.6	11.1	10.6	10.7	11.4	10.9	11.0
Additions	0.0	0.0	0.0	0.0
Notional Amount Of Derivatives Terminations	0.0	0.0	0.0	0.0
Credit Risk Contract \| Credit Default Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	11.1		11.1			11.4
Forwards
Derivative
Derivative, Notional Amount	10.3	102.9	10.3	102.9	7.6	8.9	0.0	0.0
Additions	2.7	358.9	2.7	382.4
Notional Amount Of Derivatives Terminations	0.0	256.0	1.3	279.1
Forwards \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	10.3		10.3			8.9
Forwards \| Not Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	0.0		0.0
Forwards \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Additions				350.0
Notional Amount Of Derivatives Terminations				256.0
Derivative, Loss on Derivative				0.6
Derivative, Cost of Hedge				3.8
Foreign Exchange Contracts \| Not Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	24.1		24.1			22.4
Foreign Exchange Contracts \| Cash Flow Hedging \| Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	5.3		5.3			6.6
Foreign Exchange Contracts \| Fair Value Hedging \| Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	4.6		4.6			5.0
Credit Exposure
Derivative
Credit Exposure on Derivatives						0.0
Receive Fixed/Pay Fixed \| Interest Rate Swaps
Derivative
Derivative, Notional Amount	700.2	567.3	700.2	567.3	663.1	611.1	558.8	538.2
Additions	40.4	22.0	92.4	121.6
Notional Amount Of Derivatives Terminations	3.3	13.5	3.3	92.5
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	148.2		148.2			148.2
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	210.2		210.2			213.5
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	341.8		341.8			249.4
Receive Fixed/Pay Variable \| Interest Rate Swaps
Derivative
Derivative, Notional Amount	0.0	250.0	0.0	250.0	$ 250.0	250.0	$ 250.0	$ 250.0
Additions	0.0	0.0	0.0	0.0
Notional Amount Of Derivatives Terminations	$ 250.0	$ 0.0	$ 250.0	$ 0.0
Receive Fixed/Pay Variable \| Interest Rate Swaps \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount						$ 250.0