Premium Income by Major Line of Business within Each 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
Operating Segments
Segment Reporting Information
Premium Income	2,378.3	2,371.4
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,525.8	1,527.7
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	457.7	463.0
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	215.2	203.2
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	410.0	414.5
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	41.4	41.7
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	115.7	109.5
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	218.7	230.4
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	67.1	65.4
Operating Segments | Unum International
Segment Reporting Information
Premium Income	174.4	164.6
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	97.1	90.8
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	27.3	30.9
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	28.0	23.9
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	22.0	19.0
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	426.4	434.7
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	240.7	249.3
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	96.6	93.8
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	89.1	91.6
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	251.7	244.4
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income	72.1	77.0
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	177.4	164.8
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 2.2	$ 2.6