Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information
Premium Income	$ 2,221.0	$ 2,142.2	$ 4,471.0	$ 4,285.1
Net Investment Income	623.6	620.5	1,225.9	1,222.9
Other Income	48.3	51.2	97.8	101.4
Operating Segments
Segment Reporting Information
Premium Income	2,221.0	2,142.2	4,471.0	4,285.1
Net Investment Income	623.6	620.5	1,225.9	1,222.9
Other Income	48.3	51.2	97.8	101.4
Adjusted Operating Revenue	2,892.9	2,813.9	5,794.7	5,609.4
Adjusted Operating Income (Loss)	357.4	353.5	700.7	693.4
Operating Segments \| Unum US
Segment Reporting Information
Premium Income	1,416.3	1,357.7	2,855.1	2,718.1
Net Investment Income	196.5	205.6	390.7	408.1
Other Income	28.8	30.1	57.8	58.8
Adjusted Operating Revenue	1,641.6	1,593.4	3,303.6	3,185.0
Adjusted Operating Income (Loss)	251.1	247.8	495.0	486.9
Operating Segments \| Unum UK
Segment Reporting Information
Premium Income	138.9	126.8	278.5	248.1
Net Investment Income	32.1	33.2	59.7	59.8
Other Income	0.0	0.0	0.0	0.0
Adjusted Operating Revenue	171.0	160.0	338.2	307.9
Adjusted Operating Income (Loss)	27.6	28.9	57.4	55.5
Operating Segments \| Colonial Life
Segment Reporting Information
Premium Income	395.4	376.3	793.7	750.6
Net Investment Income	40.2	36.7	77.5	71.8
Other Income	0.3	0.2	0.6	0.5
Adjusted Operating Revenue	435.9	413.2	871.8	822.9
Adjusted Operating Income (Loss)	84.6	81.8	165.6	164.2
Operating Segments \| Closed Block
Segment Reporting Information
Premium Income	270.4	281.4	543.7	568.3
Net Investment Income	345.6	340.0	683.3	675.3
Other Income	18.9	20.2	37.9	41.0
Adjusted Operating Revenue	634.9	641.6	1,264.9	1,284.6
Adjusted Operating Income (Loss)	29.6	32.6	58.5	64.2
Operating Segments \| Corporate and Other
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	9.2	5.0	14.7	7.9
Other Income	0.3	0.7	1.5	1.1
Adjusted Operating Revenue	9.5	5.7	16.2	9.0
Adjusted Operating Income (Loss)	$ (35.5)	$ (37.6)	$ (75.8)	$ (77.4)

Adjusted Operating Revenue

No definition available.

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

- Definition

Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

+ Details

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

- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

+ Details

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

- Definition

Amount of revenue and income classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.4)
-URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263

+ Details

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

- Definition

Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(1))
-URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 2
-Subparagraph (SX 210.12-17(Column E))
-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(13)(c))
-URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

+ Details

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

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

- Details

Name:	srt_ConsolidationItemsAxis=us-gaap_OperatingSegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=unm_UnumUsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=unm_UnumUkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=unm_ColonialLifeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=unm_ClosedBlockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateAndOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: