Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information
Premium Income	$ 2,525.9	$ 2,388.9	$ 7,494.3	$ 7,206.3
Operating Segments
Segment Reporting Information
Premium Income	2,525.9	2,388.9	7,494.3	7,206.3
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,657.7	1,556.8	4,908.7	4,670.8
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	518.5	480.9	1,539.3	1,419.2
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	256.5	232.2	753.0	685.9
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	423.0	417.7	1,254.3	1,249.9
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	44.1	43.8	131.2	129.7
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	209.5	196.2	637.4	629.2
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	136.7	118.9	386.7	350.3
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	69.4	67.1	206.8	206.6
Operating Segments | Unum International
Segment Reporting Information
Premium Income	210.6	173.3	607.1	540.5
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	101.1	89.8	297.5	287.5
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	44.2	35.0	124.5	100.5
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	34.6	27.4	99.9	85.8
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	30.7	21.1	85.2	66.7
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	431.2	423.3	1,291.3	1,281.6
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	236.7	236.1	708.0	714.0
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	106.2	100.2	318.1	303.2
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	88.3	87.0	265.2	264.4
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	226.4	235.5	687.2	713.4
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	172.1	174.5	521.2	523.0
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 54.3	$ 61.0	$ 166.0	$ 190.4