Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information
Premium Income	$ 2,628.8	$ 2,525.9	$ 7,866.3	$ 7,494.3
Net Investment Income	527.8	526.0	1,586.4	1,565.9
Other Income	73.3	71.6	222.5	210.6
Adjusted Operating Revenue	3,229.9	3,123.5	9,675.2	9,270.8
Adjusted Operating Income (Loss)	501.8	490.2	1,539.5	1,478.1
Operating Segments
Segment Reporting Information
Premium Income	2,628.8	2,525.9	7,866.3	7,494.3
Net Investment Income	527.8	526.0	1,586.4	1,565.9
Other Income	73.3	71.6	222.5	210.6
Adjusted Operating Revenue	3,229.9	3,123.5	9,675.2	9,270.8
Adjusted Operating Income (Loss)	501.8	490.2	1,539.5	1,478.1
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,723.5	1,657.7	5,161.8	4,908.7
Net Investment Income	161.0	166.2	476.1	481.5
Other Income	60.1	58.0	178.9	166.1
Adjusted Operating Revenue	1,944.6	1,881.9	5,816.8	5,556.3
Adjusted Operating Income (Loss)	363.3	357.8	1,106.0	1,013.4
Operating Segments | Unum International
Segment Reporting Information
Premium Income	246.6	210.6	707.1	607.1
Net Investment Income	30.4	27.3	94.5	104.0
Other Income	0.4	0.5	1.2	1.0
Adjusted Operating Revenue	277.4	238.4	802.8	712.1
Adjusted Operating Income (Loss)	40.3	36.8	120.2	118.7
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	441.9	431.2	1,335.0	1,291.3
Net Investment Income	39.6	39.3	119.4	114.6
Other Income	0.4	0.3	3.6	0.9
Adjusted Operating Revenue	481.9	470.8	1,458.0	1,406.8
Adjusted Operating Income (Loss)	113.4	102.9	344.0	312.3
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	216.8	226.4	662.4	687.2
Net Investment Income	284.3	274.9	851.6	796.0
Other Income	12.4	12.8	37.7	40.1
Adjusted Operating Revenue	513.5	514.1	1,551.7	1,523.3
Adjusted Operating Income (Loss)	34.2	34.2	110.1	143.6
Operating Segments | Corporate
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	12.5	18.3	44.8	69.8
Other Income	0.0	0.0	1.1	2.5
Adjusted Operating Revenue	12.5	18.3	45.9	72.3
Adjusted Operating Income (Loss)	$ (49.4)	$ (41.5)	$ (140.8)	$ (109.9)