Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information
Premium Income	$ 2,417.3	$ 2,374.4	$ 4,820.6	$ 4,752.7
Net Investment Income	559.0	563.5	1,086.2	1,112.2
Other Income	68.7	54.2	134.5	114.6
Operating Segments
Segment Reporting Information
Premium Income	2,417.3	2,374.4	4,820.6	4,752.7
Net Investment Income	559.0	563.5	1,086.2	1,112.2
Other Income	68.7	54.2	134.5	114.6
Adjusted Operating Revenue	3,045.0	2,992.1	6,041.3	5,979.5
Adjusted Operating Income (Loss)	463.8	362.6	806.4	636.1
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,572.3	1,522.1	3,117.2	3,047.9
Net Investment Income	167.8	183.6	338.8	363.3
Other Income	50.7	41.3	97.8	81.7
Adjusted Operating Revenue	1,790.8	1,747.0	3,553.8	3,492.9
Adjusted Operating Income (Loss)	295.4	179.3	467.0	295.0
Operating Segments | Unum International
Segment Reporting Information
Premium Income	179.4	183.5	367.2	357.9
Net Investment Income	50.8	35.7	85.3	61.7
Other Income	0.3	0.1	0.5	0.2
Adjusted Operating Revenue	230.5	219.3	453.0	419.8
Adjusted Operating Income (Loss)	24.9	24.8	52.1	51.2
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	427.6	419.7	858.3	846.1
Net Investment Income	38.7	41.6	76.8	79.3
Other Income	0.2	0.3	0.5	0.5
Adjusted Operating Revenue	466.5	461.6	935.6	925.9
Adjusted Operating Income (Loss)	101.1	95.8	191.2	169.1
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	238.0	249.1	477.9	500.8
Net Investment Income	291.5	294.7	566.3	591.9
Other Income	16.5	12.1	32.5	30.5
Adjusted Operating Revenue	546.0	555.9	1,076.7	1,123.2
Adjusted Operating Income (Loss)	79.3	111.2	173.4	208.2
Operating Segments | Corporate
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	10.2	7.9	19.0	16.0
Other Income	1.0	0.4	3.2	1.7
Adjusted Operating Revenue	11.2	8.3	22.2	17.7
Adjusted Operating Income (Loss)	$ (36.9)	$ (48.5)	$ (77.3)	$ (87.4)