Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information
Premium Income	$ 2,459.3	$ 2,401.4
Operating Segments
Segment Reporting Information
Premium Income	2,459.3	2,401.4
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,609.6	1,543.0
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	504.7	463.9
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	240.3	221.6
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	413.1	412.6
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	43.5	42.1
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	214.5	218.6
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	124.2	113.6
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	69.3	70.6
Operating Segments | Unum International
Segment Reporting Information
Premium Income	188.6	187.8
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	91.7	103.4
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	39.2	32.2
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	31.8	29.0
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	25.9	23.2
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	429.5	430.7
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	235.7	239.7
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	105.3	101.7
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	88.5	89.3
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	231.6	239.9
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	175.1	174.8
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 56.5	$ 65.1