Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information
Premium Income	$ 2,391.7	$ 2,353.7	$ 7,212.3	$ 7,106.4
Net Investment Income	511.6	550.2	1,597.8	1,662.4
Other Income	63.0	65.9	197.5	180.5
Operating Segments
Segment Reporting Information
Premium Income	2,391.7	2,353.7	7,212.3	7,106.4
Net Investment Income	511.6	550.2	1,597.8	1,662.4
Other Income	63.0	65.9	197.5	180.5
Adjusted Operating Revenue	2,966.3	2,969.8	9,007.6	8,949.3
Adjusted Operating Income (Loss)	379.9	260.4	1,186.3	896.5
Operating Segments \| Unum US
Segment Reporting Information
Premium Income	1,559.6	1,500.8	4,676.8	4,548.7
Net Investment Income	170.6	176.2	509.4	539.5
Other Income	49.0	43.5	146.8	125.2
Adjusted Operating Revenue	1,779.2	1,720.5	5,333.0	5,213.4
Adjusted Operating Income (Loss)	275.0	88.5	742.0	383.5
Operating Segments \| Unum International
Segment Reporting Information
Premium Income	173.3	181.6	540.5	539.5
Net Investment Income	37.0	33.1	122.3	94.8
Other Income	0.2	0.4	0.7	0.6
Adjusted Operating Revenue	210.5	215.1	663.5	634.9
Adjusted Operating Income (Loss)	29.9	27.4	82.0	78.6
Operating Segments \| Colonial Life
Segment Reporting Information
Premium Income	423.3	420.8	1,281.6	1,266.9
Net Investment Income	38.6	51.8	115.4	131.1
Other Income	0.3	0.3	0.8	0.8
Adjusted Operating Revenue	462.2	472.9	1,397.8	1,398.8
Adjusted Operating Income (Loss)	90.4	80.1	281.6	249.2
Operating Segments \| Closed Block
Segment Reporting Information
Premium Income	235.5	250.5	713.4	751.3
Net Investment Income	251.4	284.6	817.7	876.5
Other Income	13.1	19.3	45.6	49.8
Adjusted Operating Revenue	500.0	554.4	1,576.7	1,677.6
Adjusted Operating Income (Loss)	34.1	109.8	207.5	318.0
Operating Segments \| Corporate
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	14.0	4.5	33.0	20.5
Other Income	0.4	2.4	3.6	4.1
Adjusted Operating Revenue	14.4	6.9	36.6	24.6
Adjusted Operating Income (Loss)	$ (49.5)	$ (45.4)	$ (126.8)	$ (132.8)

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