Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information
Premium Income	$ 2,525.9	$ 2,388.9	$ 7,494.3	$ 7,206.3
Net Investment Income	526.0	511.6	1,565.9	1,597.8
Other Income	71.6	63.0	210.6	197.5
Operating Segments
Segment Reporting Information
Premium Income	2,525.9	2,388.9	7,494.3	7,206.3
Net Investment Income	526.0	511.6	1,565.9	1,597.8
Other Income	71.6	63.0	210.6	197.5
Adjusted Operating Revenue	3,123.5	2,963.5	9,270.8	9,001.6
Adjusted Operating Income (Loss)	490.2	416.2	1,478.1	1,218.1
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,657.7	1,556.8	4,908.7	4,670.8
Net Investment Income	166.2	170.6	481.5	509.4
Other Income	58.0	49.0	166.1	146.8
Adjusted Operating Revenue	1,881.9	1,776.4	5,556.3	5,327.0
Adjusted Operating Income (Loss)	357.8	280.8	1,013.4	741.0
Operating Segments | Unum International
Segment Reporting Information
Premium Income	210.6	173.3	607.1	540.5
Net Investment Income	27.3	37.0	104.0	122.3
Other Income	0.5	0.2	1.0	0.7
Adjusted Operating Revenue	238.4	210.5	712.1	663.5
Adjusted Operating Income (Loss)	36.8	24.9	118.7	78.9
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	431.2	423.3	1,291.3	1,281.6
Net Investment Income	39.3	38.6	114.6	115.4
Other Income	0.3	0.3	0.9	0.8
Adjusted Operating Revenue	470.8	462.2	1,406.8	1,397.8
Adjusted Operating Income (Loss)	102.9	117.9	312.3	317.4
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	226.4	235.5	687.2	713.4
Net Investment Income	274.9	251.4	796.0	817.7
Other Income	12.8	13.1	40.1	45.6
Adjusted Operating Revenue	514.1	500.0	1,523.3	1,576.7
Adjusted Operating Income (Loss)	34.2	42.1	143.6	207.6
Operating Segments | Corporate
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	18.3	14.0	69.8	33.0
Other Income	0.0	0.4	2.5	3.6
Adjusted Operating Revenue	18.3	14.4	72.3	36.6
Adjusted Operating Income (Loss)	$ (41.5)	$ (49.5)	$ (109.9)	$ (126.8)