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Segment Information Segments - Additional Information (Detail)
$ in Millions
12 Months Ended
Dec. 31, 2019
USD ($)
Integer
Rate
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Segment Reporting [Abstract]      
Number of Operating Segments | Integer 3    
Supplementary Insurance Information, by Segment      
Goodwill $ 351.7 $ 350.3  
Unum US      
Supplementary Insurance Information, by Segment      
Goodwill 280.0 280.0  
Unum International      
Supplementary Insurance Information, by Segment      
Goodwill 44.0 42.6  
Colonial Life      
Supplementary Insurance Information, by Segment      
Goodwill $ 27.7 27.7  
Jaimini Health, Inc.      
Supplementary Insurance Information, by Segment      
Percentage Ownership of Acquired Business | Rate 100.00%    
Name of Acquired Businesses Jaimini Health, Inc. (Jaimini Health)    
Unum Poland      
Supplementary Insurance Information, by Segment      
Percentage Ownership of Acquired Business | Rate 100.00%    
Name of Acquired Businesses Unum Poland    
Leavelogic Inc.      
Supplementary Insurance Information, by Segment      
Percentage Ownership of Acquired Business | Rate 100.00%    
Name of Acquired Businesses Leavelogic, Inc (Leavelogic)    
Series of Individually Immaterial Business Acquisitions      
Supplementary Insurance Information, by Segment      
Business Acquisition, Revenue Reported by Acquired Entity for Last Annual Period     $ 61.0
Assets of Acquiree at Acquisition Date   344.0  
Liabilities of Acquiree at Acquisition Date   206.0  
Total Cash Purchase Price to Acquire Business   151.0  
Goodwill   $ 13.0