Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information
Premium Income	$ 2,610.3	$ 2,459.3
Net Investment Income	513.5	508.8
Other Income	77.7	67.9
Operating Segments
Segment Reporting Information
Premium Income	2,610.3	2,459.3
Net Investment Income	513.5	508.8
Other Income	77.7	67.9
Adjusted Operating Revenue	3,201.5	3,036.0
Adjusted Operating Income (Loss)	514.5	469.5
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,707.4	1,609.6
Net Investment Income	157.0	157.3
Other Income	60.6	53.6
Adjusted Operating Revenue	1,925.0	1,820.5
Adjusted Operating Income (Loss)	385.2	312.5
Operating Segments | Unum International
Segment Reporting Information
Premium Income	231.7	188.6
Net Investment Income	26.1	30.9
Other Income	0.3	0.4
Adjusted Operating Revenue	258.1	219.9
Adjusted Operating Income (Loss)	37.4	38.4
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	446.9	429.5
Net Investment Income	39.3	37.3
Other Income	3.0	0.2
Adjusted Operating Revenue	489.2	467.0
Adjusted Operating Income (Loss)	113.7	93.9
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	224.3	231.6
Net Investment Income	273.1	257.2
Other Income	13.1	13.5
Adjusted Operating Revenue	510.5	502.3
Adjusted Operating Income (Loss)	24.3	58.2
Operating Segments | Corporate
Segment Reporting Information
Premium Income	0.0	0.0
Net Investment Income	18.0	26.1
Other Income	0.7	0.2
Adjusted Operating Revenue	18.7	26.3
Adjusted Operating Income (Loss)	$ (46.1)	$ (33.5)