Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information
Premium Income	$ 2,343.1	$ 2,221.0	$ 4,681.8	$ 4,471.0
Net Investment Income	624.9	623.6	1,219.6	1,225.9
Other Income	56.0	48.3	109.1	97.8
Operating Segments
Segment Reporting Information
Premium Income	2,343.1	2,221.0	4,681.8	4,471.0
Net Investment Income	624.9	623.6	1,219.6	1,225.9
Other Income	56.0	48.3	109.1	97.8
Adjusted Operating Revenue	3,024.0	2,892.9	6,010.5	5,794.7
Adjusted Operating Income (Loss)	359.3	357.4	711.5	700.7
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,504.5	1,416.3	3,006.4	2,855.1
Net Investment Income	184.1	196.5	366.2	390.7
Other Income	36.8	28.8	71.3	57.8
Adjusted Operating Revenue	1,725.4	1,641.6	3,443.9	3,303.6
Adjusted Operating Income (Loss)	254.3	251.1	506.6	495.0
Operating Segments | Unum International
Segment Reporting Information
Premium Income	158.6	138.9	312.6	278.5
Net Investment Income	44.8	32.1	69.6	59.7
Other Income	0.3	0.0	0.3	0.0
Adjusted Operating Revenue	203.7	171.0	382.5	338.2
Adjusted Operating Income (Loss)	30.7	27.6	59.8	57.4
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	420.9	395.4	840.2	793.7
Net Investment Income	37.2	40.2	74.1	77.5
Other Income	0.7	0.3	1.3	0.6
Adjusted Operating Revenue	458.8	435.9	915.6	871.8
Adjusted Operating Income (Loss)	84.4	84.6	169.6	165.6
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	259.1	270.4	522.6	543.7
Net Investment Income	354.5	345.6	701.1	683.3
Other Income	16.5	18.9	34.5	37.9
Adjusted Operating Revenue	630.1	634.9	1,258.2	1,264.9
Adjusted Operating Income (Loss)	33.7	29.6	64.7	58.5
Operating Segments | Corporate and Other
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	4.3	9.2	8.6	14.7
Other Income	1.7	0.3	1.7	1.5
Adjusted Operating Revenue	6.0	9.5	10.3	16.2
Adjusted Operating Income (Loss)	$ (43.8)	$ (35.5)	$ (89.2)	$ (75.8)