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Segment Information Segments - Additional Information (Detail)
$ in Millions
12 Months Ended
Dec. 31, 2022
USD ($)
Integer
Dec. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Segment Reporting [Abstract]      
Number of Operating Segments | Integer 3    
Supplementary Insurance Information, by Segment      
Goodwill $ 347.6 $ 352.2  
Operating Segments      
Supplementary Insurance Information, by Segment      
Capitalized Computer Software, Impairments 0.0 12.1 $ 0.0
Costs Related to Organizational Design Update     $ (23.3)
Unum US      
Supplementary Insurance Information, by Segment      
Goodwill 280.0 280.0  
Unum International      
Supplementary Insurance Information, by Segment      
Goodwill 39.9 44.5  
Colonial Life      
Supplementary Insurance Information, by Segment      
Goodwill $ 27.7 $ 27.7