Reinsurance - Additional Information (Detail)	12 Months Ended
Dec. 31, 2017
Number of companies that make up majority of reinsurance recoverable	13
AM Best A Rating or Better \| Reinsurer Concentration Risk \| Reinsurance Recoverable \| Top 13 Reinsurers
Percentage of Concentration Risk	91.00%
AM Best, A- Rating \| Reinsurer Concentration Risk \| Reinsurance Recoverable \| Other Reinsurers
Percentage of Concentration Risk	8.00%
AM Best, Below A- or Not Rated \| Reinsurer Concentration Risk \| Reinsurance Recoverable \| Other Reinsurers
Percentage of Concentration Risk	1.00%
Individual Disability \| Unum US
Reinsurance Retention Policy, Reinsured Risk, Percentage	30.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

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