Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information
Premium Income	$ 2,331.2	$ 2,251.8	$ 7,013.0	$ 6,722.8
Net Investment Income	599.4	619.2	1,819.0	1,845.1
Other Income	55.6	50.1	164.7	147.9
Operating Segments
Segment Reporting Information
Premium Income	2,331.2	2,251.8	7,013.0	6,722.8
Net Investment Income	599.4	619.2	1,819.0	1,845.1
Other Income	55.6	50.1	164.7	147.9
Adjusted Operating Revenue	2,986.2	2,921.1	8,996.7	8,715.8
Adjusted Operating Income (Loss)	350.8	366.3	1,062.3	1,067.0
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,501.9	1,446.2	4,508.3	4,301.3
Net Investment Income	184.2	200.3	550.4	591.0
Other Income	34.8	30.7	106.1	88.5
Adjusted Operating Revenue	1,720.9	1,677.2	5,164.8	4,980.8
Adjusted Operating Income (Loss)	261.4	270.9	768.0	765.9
Operating Segments | Unum International
Segment Reporting Information
Premium Income	152.3	138.0	464.9	416.5
Net Investment Income	24.3	26.4	93.9	86.1
Other Income	0.0	0.0	0.3	0.0
Adjusted Operating Revenue	176.6	164.4	559.1	502.6
Adjusted Operating Income (Loss)	24.2	26.1	84.0	83.5
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	419.9	400.0	1,260.1	1,193.7
Net Investment Income	36.9	36.7	111.0	114.2
Other Income	0.9	0.4	2.2	1.0
Adjusted Operating Revenue	457.7	437.1	1,373.3	1,308.9
Adjusted Operating Income (Loss)	87.2	84.2	256.8	249.8
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	257.1	267.6	779.7	811.3
Net Investment Income	347.3	348.0	1,048.3	1,031.3
Other Income	18.9	18.9	53.5	56.8
Adjusted Operating Revenue	623.3	634.5	1,881.5	1,899.4
Adjusted Operating Income (Loss)	26.9	32.2	91.6	90.7
Operating Segments | Corporate and Other
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	6.7	7.8	15.4	22.5
Other Income	1.0	0.1	2.6	1.6
Adjusted Operating Revenue	7.7	7.9	18.0	24.1
Adjusted Operating Income (Loss)	$ (48.9)	$ (47.1)	$ (138.1)	$ (122.9)