Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information
Premium Income	$ 2,318.1	$ 2,331.2	$ 7,058.2	$ 7,013.0
Net Investment Income	613.2	599.4	1,767.2	1,819.0
Other Income	60.6	55.6	169.0	164.7
Operating Segments
Segment Reporting Information
Premium Income	2,318.1	2,331.2	7,058.2	7,013.0
Net Investment Income	613.2	599.4	1,767.2	1,819.0
Other Income	60.6	55.6	169.0	164.7
Adjusted Operating Revenue	2,991.9	2,986.2	8,994.4	8,996.7
Adjusted Operating Income (Loss)	318.5	350.8	981.1	1,062.3
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,483.4	1,501.9	4,533.8	4,508.3
Net Investment Income	190.7	184.2	547.2	550.4
Other Income	42.0	34.8	117.5	106.1
Adjusted Operating Revenue	1,716.1	1,720.9	5,198.5	5,164.8
Adjusted Operating Income (Loss)	188.2	261.4	681.9	768.0
Operating Segments | Unum International
Segment Reporting Information
Premium Income	165.4	152.3	487.1	464.9
Net Investment Income	26.3	24.3	79.1	93.9
Other Income	0.2	0.0	0.4	0.3
Adjusted Operating Revenue	191.9	176.6	566.6	559.1
Adjusted Operating Income (Loss)	21.4	24.2	55.9	84.0
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	419.9	419.9	1,293.2	1,260.1
Net Investment Income	43.7	36.9	118.2	111.0
Other Income	0.3	0.9	0.9	2.2
Adjusted Operating Revenue	463.9	457.7	1,412.3	1,373.3
Adjusted Operating Income (Loss)	92.2	87.2	264.2	256.8
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	249.4	257.1	744.1	779.7
Net Investment Income	351.2	347.3	1,013.6	1,048.3
Other Income	17.8	18.9	49.7	53.5
Adjusted Operating Revenue	618.4	623.3	1,807.4	1,881.5
Adjusted Operating Income (Loss)	70.8	26.9	137.2	91.6
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	1.3	6.7	9.1	15.4
Other Income	0.3	1.0	0.5	2.6
Adjusted Operating Revenue	1.6	7.7	9.6	18.0
Adjusted Operating Income (Loss)	$ (54.1)	$ (48.9)	$ (158.1)	$ (138.1)