Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information
Premium Income	$ 2,338.7	$ 2,250.0
Net Investment Income	594.7	602.3
Other Income	53.1	49.5
Operating Segments
Segment Reporting Information
Premium Income	2,338.7	2,250.0
Net Investment Income	594.7	602.3
Other Income	53.1	49.5
Adjusted Operating Revenue	2,986.5	2,901.8
Adjusted Operating Income (Loss)	352.2	343.3
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,501.9	1,438.8
Net Investment Income	182.1	194.2
Other Income	34.5	29.0
Adjusted Operating Revenue	1,718.5	1,662.0
Adjusted Operating Income (Loss)	252.3	243.9
Operating Segments | Unum International
Segment Reporting Information
Premium Income	154.0	139.6
Net Investment Income	24.8	27.6
Other Income	0.0	0.0
Adjusted Operating Revenue	178.8	167.2
Adjusted Operating Income (Loss)	29.1	29.8
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	419.3	398.3
Net Investment Income	36.9	37.3
Other Income	0.6	0.3
Adjusted Operating Revenue	456.8	435.9
Adjusted Operating Income (Loss)	85.2	81.0
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	263.5	273.3
Net Investment Income	346.6	337.7
Other Income	18.0	19.0
Adjusted Operating Revenue	628.1	630.0
Adjusted Operating Income (Loss)	31.0	28.9
Operating Segments | Corporate and Other
Segment Reporting Information
Premium Income	0.0	0.0
Net Investment Income	4.3	5.5
Other Income	0.0	1.2
Adjusted Operating Revenue	4.3	6.7
Adjusted Operating Income (Loss)	$ (45.4)	$ (40.3)