Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information
Premium Income	$ 2,403.3	$ 2,378.3
Operating Segments
Segment Reporting Information
Premium Income	2,403.3	2,378.3
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,544.9	1,525.8
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	463.9	457.7
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	221.6	215.2
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	412.6	410.0
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	42.1	41.4
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	220.5	218.7
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	113.6	115.7
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	70.6	67.1
Operating Segments | Unum International
Segment Reporting Information
Premium Income	187.8	174.4
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	103.4	97.1
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	32.2	27.3
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	29.0	28.0
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	23.2	22.0
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	430.7	426.4
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	239.7	240.7
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	101.7	96.6
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	89.3	89.1
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	239.9	251.7
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income	63.4	72.1
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	174.8	177.4
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 1.7	$ 2.2