Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information
Premium Income	$ 2,378.3	$ 2,371.4
Net Investment Income	548.7	585.0
Other Income	60.4	58.7
Operating Segments
Segment Reporting Information
Premium Income	2,378.3	2,371.4
Net Investment Income	548.7	585.0
Other Income	60.4	58.7
Adjusted Operating Revenue	2,987.4	3,015.1
Adjusted Operating Income (Loss)	273.5	346.1
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,525.8	1,527.7
Net Investment Income	179.7	179.6
Other Income	40.4	40.2
Adjusted Operating Revenue	1,745.9	1,747.5
Adjusted Operating Income (Loss)	115.7	261.8
Operating Segments | Unum International
Segment Reporting Information
Premium Income	174.4	164.6
Net Investment Income	26.0	26.5
Other Income	0.1	0.0
Adjusted Operating Revenue	200.5	191.1
Adjusted Operating Income (Loss)	26.4	19.4
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	426.4	434.7
Net Investment Income	37.7	37.7
Other Income	0.2	0.3
Adjusted Operating Revenue	464.3	472.7
Adjusted Operating Income (Loss)	73.3	81.1
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	251.7	244.4
Net Investment Income	297.2	336.1
Other Income	18.4	18.2
Adjusted Operating Revenue	567.3	598.7
Adjusted Operating Income (Loss)	97.0	29.7
Operating Segments | Corporate
Segment Reporting Information
Premium Income	0.0	0.0
Net Investment Income	8.1	5.1
Other Income	1.3	0.0
Adjusted Operating Revenue	9.4	5.1
Adjusted Operating Income (Loss)	$ (38.9)	$ (45.9)