Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information
Premium Income	$ 2,627.2	$ 2,509.1	$ 5,237.5	$ 4,968.4
Net Investment Income	545.1	531.1	1,058.6	1,039.9
Other Income	71.5	71.1	149.2	139.0
Operating Segments
Segment Reporting Information
Premium Income	2,627.2	2,509.1	5,237.5	4,968.4
Net Investment Income	545.1	531.1	1,058.6	1,039.9
Other Income	71.5	71.1	149.2	139.0
Adjusted Operating Revenue	3,243.8	3,111.3	6,445.3	6,147.3
Adjusted Operating Income (Loss)	523.2	518.4	1,037.7	987.9
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,730.9	1,641.4	3,438.3	3,251.0
Net Investment Income	158.1	158.0	315.1	315.3
Other Income	58.2	54.5	118.8	108.1
Adjusted Operating Revenue	1,947.2	1,853.9	3,872.2	3,674.4
Adjusted Operating Income (Loss)	357.5	343.1	742.7	655.6
Operating Segments | Unum International
Segment Reporting Information
Premium Income	228.8	207.9	460.5	396.5
Net Investment Income	38.0	45.8	64.1	76.7
Other Income	0.5	0.1	0.8	0.5
Adjusted Operating Revenue	267.3	253.8	525.4	473.7
Adjusted Operating Income (Loss)	42.5	43.5	79.9	81.9
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	446.2	430.6	893.1	860.1
Net Investment Income	40.5	38.0	79.8	75.3
Other Income	0.2	0.4	3.2	0.6
Adjusted Operating Revenue	486.9	469.0	976.1	936.0
Adjusted Operating Income (Loss)	116.9	115.5	230.6	209.4
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	221.3	229.2	445.6	460.8
Net Investment Income	294.2	263.9	567.3	521.1
Other Income	12.2	13.8	25.3	27.3
Adjusted Operating Revenue	527.7	506.9	1,038.2	1,009.2
Adjusted Operating Income (Loss)	51.6	51.2	75.9	109.4
Operating Segments | Corporate
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	14.3	25.4	32.3	51.5
Other Income	0.4	2.3	1.1	2.5
Adjusted Operating Revenue	14.7	27.7	33.4	54.0
Adjusted Operating Income (Loss)	$ (45.3)	$ (34.9)	$ (91.4)	$ (68.4)