Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information
Premium Income	$ 2,371.4	$ 2,338.7
Net Investment Income	585.0	594.7
Other Income	58.7	53.1
Operating Segments
Segment Reporting Information
Premium Income	2,371.4	2,338.7
Net Investment Income	585.0	594.7
Other Income	58.7	53.1
Adjusted Operating Revenue	3,015.1	2,986.5
Adjusted Operating Income (Loss)	346.1	352.2
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,527.7	1,501.9
Net Investment Income	179.6	182.1
Other Income	40.2	34.5
Adjusted Operating Revenue	1,747.5	1,718.5
Adjusted Operating Income (Loss)	261.8	252.3
Operating Segments | Unum International
Segment Reporting Information
Premium Income	164.6	154.0
Net Investment Income	26.5	24.8
Other Income	0.0	0.0
Adjusted Operating Revenue	191.1	178.8
Adjusted Operating Income (Loss)	19.4	29.1
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	434.7	419.3
Net Investment Income	37.7	36.9
Other Income	0.3	0.6
Adjusted Operating Revenue	472.7	456.8
Adjusted Operating Income (Loss)	81.1	85.2
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	244.4	263.5
Net Investment Income	336.1	346.6
Other Income	18.2	18.0
Adjusted Operating Revenue	598.7	628.1
Adjusted Operating Income (Loss)	29.7	31.0
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Premium Income	0.0	0.0
Net Investment Income	5.1	4.3
Other Income	0.0	0.0
Adjusted Operating Revenue	5.1	4.3
Adjusted Operating Income (Loss)	$ (45.9)	$ (45.4)