Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information
Premium Income	$ 2,459.3	$ 2,401.4
Net Investment Income	508.8	527.2
Other Income	67.9	65.8
Operating Segments
Segment Reporting Information
Premium Income	2,459.3	2,401.4
Net Investment Income	508.8	527.2
Other Income	67.9	65.8
Adjusted Operating Revenue	3,036.0	2,994.4
Adjusted Operating Income (Loss)	469.5	335.3
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,609.6	1,543.0
Net Investment Income	157.3	171.0
Other Income	53.6	47.1
Adjusted Operating Revenue	1,820.5	1,761.1
Adjusted Operating Income (Loss)	312.5	168.3
Operating Segments | Unum International
Segment Reporting Information
Premium Income	188.6	187.8
Net Investment Income	30.9	34.5
Other Income	0.4	0.2
Adjusted Operating Revenue	219.9	222.5
Adjusted Operating Income (Loss)	38.4	25.9
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	429.5	430.7
Net Investment Income	37.3	38.1
Other Income	0.2	0.3
Adjusted Operating Revenue	467.0	469.1
Adjusted Operating Income (Loss)	93.9	102.9
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	231.6	239.9
Net Investment Income	257.2	274.8
Other Income	13.5	16.0
Adjusted Operating Revenue	502.3	530.7
Adjusted Operating Income (Loss)	58.2	78.6
Operating Segments | Corporate
Segment Reporting Information
Premium Income	0.0	0.0
Net Investment Income	26.1	8.8
Other Income	0.2	2.2
Adjusted Operating Revenue	26.3	11.0
Adjusted Operating Income (Loss)	$ (33.5)	$ (40.4)