Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information
Premium Income	$ 2,403.3	$ 2,378.3
Net Investment Income	527.2	548.7
Other Income	65.8	60.4
Operating Segments
Segment Reporting Information
Premium Income	2,403.3	2,378.3
Net Investment Income	527.2	548.7
Other Income	65.8	60.4
Adjusted Operating Revenue	2,996.3	2,987.4
Adjusted Operating Income (Loss)	342.6	273.5
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,544.9	1,525.8
Net Investment Income	171.0	179.7
Other Income	47.1	40.4
Adjusted Operating Revenue	1,763.0	1,745.9
Adjusted Operating Income (Loss)	171.6	115.7
Operating Segments | Unum International
Segment Reporting Information
Premium Income	187.8	174.4
Net Investment Income	34.5	26.0
Other Income	0.2	0.1
Adjusted Operating Revenue	222.5	200.5
Adjusted Operating Income (Loss)	27.2	26.4
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	430.7	426.4
Net Investment Income	38.1	37.7
Other Income	0.3	0.2
Adjusted Operating Revenue	469.1	464.3
Adjusted Operating Income (Loss)	90.1	73.3
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	239.9	251.7
Net Investment Income	274.8	297.2
Other Income	16.0	18.4
Adjusted Operating Revenue	530.7	567.3
Adjusted Operating Income (Loss)	94.1	97.0
Operating Segments | Corporate
Segment Reporting Information
Premium Income	0.0	0.0
Net Investment Income	8.8	8.1
Other Income	2.2	1.3
Adjusted Operating Revenue	11.0	9.4
Adjusted Operating Income (Loss)	$ (40.4)	$ (38.9)