Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information
Premium Income	$ 2,509.1	$ 2,416.0	$ 4,968.4	$ 4,817.4
Net Investment Income	531.1	559.0	1,039.9	1,086.2
Other Income	71.1	68.7	139.0	134.5
Operating Segments
Segment Reporting Information
Premium Income	2,509.1	2,416.0	4,968.4	4,817.4
Net Investment Income	531.1	559.0	1,039.9	1,086.2
Other Income	71.1	68.7	139.0	134.5
Adjusted Operating Revenue	3,111.3	3,043.7	6,147.3	6,038.1
Adjusted Operating Income (Loss)	518.4	466.6	987.9	801.9
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,641.4	1,571.0	3,251.0	3,114.0
Net Investment Income	158.0	167.8	315.3	338.8
Other Income	54.5	50.7	108.1	97.8
Adjusted Operating Revenue	1,853.9	1,789.5	3,674.4	3,550.6
Adjusted Operating Income (Loss)	343.1	291.9	655.6	460.2
Operating Segments | Unum International
Segment Reporting Information
Premium Income	207.9	179.4	396.5	367.2
Net Investment Income	45.8	50.8	76.7	85.3
Other Income	0.1	0.3	0.5	0.5
Adjusted Operating Revenue	253.8	230.5	473.7	453.0
Adjusted Operating Income (Loss)	43.5	28.1	81.9	54.0
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	430.6	427.6	860.1	858.3
Net Investment Income	38.0	38.7	75.3	76.8
Other Income	0.4	0.2	0.6	0.5
Adjusted Operating Revenue	469.0	466.5	936.0	935.6
Adjusted Operating Income (Loss)	115.5	96.6	209.4	199.5
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	229.2	238.0	460.8	477.9
Net Investment Income	263.9	291.5	521.1	566.3
Other Income	13.8	16.5	27.3	32.5
Adjusted Operating Revenue	506.9	546.0	1,009.2	1,076.7
Adjusted Operating Income (Loss)	51.2	86.9	109.4	165.5
Operating Segments | Corporate
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	25.4	10.2	51.5	19.0
Other Income	2.3	1.0	2.5	3.2
Adjusted Operating Revenue	27.7	11.2	54.0	22.2
Adjusted Operating Income (Loss)	$ (34.9)	$ (36.9)	$ (68.4)	$ (77.3)