Selected Operating Statement Data by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information
Premium Income	$ 2,006.3us-gaap_PremiumsEarnedNet	$ 1,938.5us-gaap_PremiumsEarnedNet
Net Investment Income	602.0us-gaap_NetInvestmentIncome	615.8us-gaap_NetInvestmentIncome
Other Income	54.4us-gaap_OtherIncome	53.8us-gaap_OtherIncome
Operating Segments
Segment Reporting Information
Premium Income	2,006.3	1,938.5
Net Investment Income	602.0	615.8
Other Income	54.4	53.8
Operating Revenue	2,662.7	2,608.1
Operating Income (Loss)	321.3	324.9
Operating Segments \| Unum US
Segment Reporting Information
Premium Income	1,230.2	1,152.5
Net Investment Income	215.0	223.1
Other Income	30.9	30.9
Operating Revenue	1,476.1	1,406.5
Operating Income (Loss)	214.3	207.5
Operating Segments \| Unum UK
Segment Reporting Information
Premium Income	139.0	151.7
Net Investment Income	23.5	33.6
Other Income	0	(0.1)
Operating Revenue	162.5	185.2
Operating Income (Loss)	32.6	36.5
Operating Segments \| Colonial Life
Segment Reporting Information
Premium Income	332.2	316.4
Net Investment Income	37.0	36.6
Other Income	0	(0.1)
Operating Revenue	369.2	352.9
Operating Income (Loss)	77.6	79.1
Operating Segments \| Closed Block
Segment Reporting Information
Premium Income	304.9	317.9
Net Investment Income	320.4	315.5
Other Income	23.2	21.6
Operating Revenue	648.5	655.0
Operating Income (Loss)	26.7	28.3
Operating Segments \| Corporate and Other
Segment Reporting Information
Premium Income	0	0
Net Investment Income	6.1	7.0
Other Income	0.3	1.5
Operating Revenue	6.4	8.5
Operating Income (Loss)	$ (29.9)	$ (26.5)

Operating Revenue

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- 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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 16

-Article 12

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.2)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 2

-Article 7

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Reflects the sum of all other revenue and income recognized by the entity in the period not otherwise specified in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.4)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 4

-Article 7

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- 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/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 235

-Section S99

-Paragraph 2

-Subparagraph (SX 210.12-17)

-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.1)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 1

-Article 7

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.13(c))

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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