Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information
Premium Income	$ 2,374.4	$ 2,368.7	$ 4,752.7	$ 4,740.1
Net Investment Income	563.5	569.0	1,112.2	1,154.0
Other Income	54.2	49.7	114.6	108.4
Operating Segments
Segment Reporting Information
Premium Income	2,374.4	2,368.7	4,752.7	4,740.1
Net Investment Income	563.5	569.0	1,112.2	1,154.0
Other Income	54.2	49.7	114.6	108.4
Adjusted Operating Revenue	2,992.1	2,987.4	5,979.5	6,002.5
Adjusted Operating Income (Loss)	362.6	316.5	636.1	662.6
Operating Segments \| Unum US
Segment Reporting Information
Premium Income	1,522.1	1,522.7	3,047.9	3,050.4
Net Investment Income	183.6	176.9	363.3	356.5
Other Income	41.3	35.3	81.7	75.5
Adjusted Operating Revenue	1,747.0	1,734.9	3,492.9	3,482.4
Adjusted Operating Income (Loss)	179.3	231.9	295.0	493.7
Operating Segments \| Unum International
Segment Reporting Information
Premium Income	183.5	157.1	357.9	321.7
Net Investment Income	35.7	26.3	61.7	52.8
Other Income	0.1	0.2	0.2	0.2
Adjusted Operating Revenue	219.3	183.6	419.8	374.7
Adjusted Operating Income (Loss)	24.8	15.1	51.2	34.5
Operating Segments \| Colonial Life
Segment Reporting Information
Premium Income	419.7	438.6	846.1	873.3
Net Investment Income	41.6	36.8	79.3	74.5
Other Income	0.3	0.3	0.5	0.6
Adjusted Operating Revenue	461.6	475.7	925.9	948.4
Adjusted Operating Income (Loss)	95.8	90.9	169.1	172.0
Operating Segments \| Closed Block
Segment Reporting Information
Premium Income	249.1	250.3	500.8	494.7
Net Investment Income	294.7	326.3	591.9	662.4
Other Income	12.1	13.7	30.5	31.9
Adjusted Operating Revenue	555.9	590.3	1,123.2	1,189.0
Adjusted Operating Income (Loss)	111.2	36.7	208.2	66.4
Operating Segments \| Corporate
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	7.9	2.7	16.0	7.8
Other Income	0.4	0.2	1.7	0.2
Adjusted Operating Revenue	8.3	2.9	17.7	8.0
Adjusted Operating Income (Loss)	$ (48.5)	$ (58.1)	$ (87.4)	$ (104.0)

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