Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information
Premium Income	$ 2,353.7	$ 2,318.1	$ 7,106.4	$ 7,058.2
Net Investment Income	550.2	613.2	1,662.4	1,767.2
Other Income	65.9	60.6	180.5	169.0
Operating Segments
Segment Reporting Information
Premium Income	2,353.7	2,318.1	7,106.4	7,058.2
Net Investment Income	550.2	613.2	1,662.4	1,767.2
Other Income	65.9	60.6	180.5	169.0
Adjusted Operating Revenue	2,969.8	2,991.9	8,949.3	8,994.4
Adjusted Operating Income (Loss)	260.4	318.5	896.5	981.1
Operating Segments \| Unum US
Segment Reporting Information
Premium Income	1,500.8	1,483.4	4,548.7	4,533.8
Net Investment Income	176.2	190.7	539.5	547.2
Other Income	43.5	42.0	125.2	117.5
Adjusted Operating Revenue	1,720.5	1,716.1	5,213.4	5,198.5
Adjusted Operating Income (Loss)	88.5	188.2	383.5	681.9
Operating Segments \| Unum International
Segment Reporting Information
Premium Income	181.6	165.4	539.5	487.1
Net Investment Income	33.1	26.3	94.8	79.1
Other Income	0.4	0.2	0.6	0.4
Adjusted Operating Revenue	215.1	191.9	634.9	566.6
Adjusted Operating Income (Loss)	27.4	21.4	78.6	55.9
Operating Segments \| Colonial Life
Segment Reporting Information
Premium Income	420.8	419.9	1,266.9	1,293.2
Net Investment Income	51.8	43.7	131.1	118.2
Other Income	0.3	0.3	0.8	0.9
Adjusted Operating Revenue	472.9	463.9	1,398.8	1,412.3
Adjusted Operating Income (Loss)	80.1	92.2	249.2	264.2
Operating Segments \| Closed Block
Segment Reporting Information
Premium Income	250.5	249.4	751.3	744.1
Net Investment Income	284.6	351.2	876.5	1,013.6
Other Income	19.3	17.8	49.8	49.7
Adjusted Operating Revenue	554.4	618.4	1,677.6	1,807.4
Adjusted Operating Income (Loss)	109.8	70.8	318.0	137.2
Operating Segments \| Corporate
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	4.5	1.3	20.5	9.1
Other Income	2.4	0.3	4.1	0.5
Adjusted Operating Revenue	6.9	1.6	24.6	9.6
Adjusted Operating Income (Loss)	$ (45.4)	$ (54.1)	$ (132.8)	$ (158.1)

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