Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information
Premium Income	$ 2,368.7	$ 2,343.1	$ 4,740.1	$ 4,681.8
Net Investment Income	569.0	624.9	1,154.0	1,219.6
Other Income	49.7	56.0	108.4	109.1
Operating Segments
Segment Reporting Information
Premium Income	2,368.7	2,343.1	4,740.1	4,681.8
Net Investment Income	569.0	624.9	1,154.0	1,219.6
Other Income	49.7	56.0	108.4	109.1
Adjusted Operating Revenue	2,987.4	3,024.0	6,002.5	6,010.5
Adjusted Operating Income (Loss)	316.5	359.3	662.6	711.5
Operating Segments \| Unum US
Segment Reporting Information
Premium Income	1,522.7	1,504.5	3,050.4	3,006.4
Net Investment Income	176.9	184.1	356.5	366.2
Other Income	35.3	36.8	75.5	71.3
Adjusted Operating Revenue	1,734.9	1,725.4	3,482.4	3,443.9
Adjusted Operating Income (Loss)	231.9	254.3	493.7	506.6
Operating Segments \| Unum International
Segment Reporting Information
Premium Income	157.1	158.6	321.7	312.6
Net Investment Income	26.3	44.8	52.8	69.6
Other Income	0.2	0.3	0.2	0.3
Adjusted Operating Revenue	183.6	203.7	374.7	382.5
Adjusted Operating Income (Loss)	15.1	30.7	34.5	59.8
Operating Segments \| Colonial Life
Segment Reporting Information
Premium Income	438.6	420.9	873.3	840.2
Net Investment Income	36.8	37.2	74.5	74.1
Other Income	0.3	0.7	0.6	1.3
Adjusted Operating Revenue	475.7	458.8	948.4	915.6
Adjusted Operating Income (Loss)	90.9	84.4	172.0	169.6
Operating Segments \| Closed Block
Segment Reporting Information
Premium Income	250.3	259.1	494.7	522.6
Net Investment Income	326.3	354.5	662.4	701.1
Other Income	13.7	16.5	31.9	34.5
Adjusted Operating Revenue	590.3	630.1	1,189.0	1,258.2
Adjusted Operating Income (Loss)	36.7	33.7	66.4	64.7
Operating Segments \| Corporate and Other Segment
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	2.7	4.3	7.8	8.6
Other Income	0.2	1.7	0.2	1.7
Adjusted Operating Revenue	2.9	6.0	8.0	10.3
Adjusted Operating Income (Loss)	$ (58.1)	$ (43.8)	$ (104.0)	$ (89.2)

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