Location of Gains and Losses on Derivative Financial Instruments Designated as Cash Flow Hedging Instruments (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ 12.3	$ 15.8
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	(4.5)	(24.3)
Foreign Exchange Contracts \| Net Investment Income
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(0.3)	(0.1)
Foreign Exchange Contracts \| Net Realized Investments Gain (Loss)
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1.2)	0.0
Interest Rate Swaps \| Net Investment Income
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	14.3	13.2
Interest Rate Swaps \| Net Realized Investments Gain (Loss)
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	0.0	3.2
Interest Rate Swaps \| Interest Expense
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ (0.5)	$ (0.5)

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.

No definition available.

+ Details

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

- Definition

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4D
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624177-113959

+ Details

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

- Definition

The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

+ References

+ Details

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

- Details

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

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_InvestmentIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=unm_NetRealizedInvestmentsGainLossMember
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_IncomeStatementLocationAxis=us-gaap_InterestExpenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: