Segment Information Segments - Additional Information (Detail) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2016 USD ($) Integer Rate	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired \| Rate	100.00%
Business Combination, Contingent Consideration, Liability, Noncurrent	$ 10.0
Business Acquisition, Revenue Reported by Acquired Entity for Last Annual Period		$ 129.9
Total Acquiree Assets Purchased	93.0
Total Acquiree Liabilities Purchased	56.0
Goodwill	$ 336.8	$ 230.9
Number of Operating Segments \| Integer	3
Payable due 18 Months After Acquisition
Business Acquisition [Line Items]
Business Combination, Contingent Consideration, Liability, Noncurrent	$ 5.0
Payable due 24 Months After Acquisition
Business Acquisition [Line Items]
Business Combination, Contingent Consideration, Liability, Noncurrent	5.0
Unum US and Colonial
Business Acquisition [Line Items]
Goodwill	$ 111.0
Unum US
Business Acquisition [Line Items]
Percent of Goodwill Allocated to a Business Segment \| Rate	75.00%
Colonial Life
Business Acquisition [Line Items]
Percent of Goodwill Allocated to a Business Segment \| Rate	25.00%

Percent of Goodwill Allocated to a Business Segment

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- Definition

Total Acquiree Assets Purchased

+ References

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- Definition

Total Acquiree Liabilities Purchased

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- 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.

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- Definition

Percentage of voting equity interests acquired at the acquisition date in the business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65885119&loc=d3e1392-128463

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- Definition

Total revenue reported by the acquired entity for its previous full fiscal year before the business acquisition.

+ References

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- Definition

Amount of liability recognized arising from contingent consideration in a business combination, expected to be settled beyond one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 25
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476

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-Name Accounting Standards Codification
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-Section 35
-Paragraph 1
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- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
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-Section 45
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-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

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-Name Accounting Standards Codification
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- Definition

Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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