Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information
Premium Income	$ 2,509.1	$ 2,416.0	$ 4,968.4	$ 4,817.4
Operating Segments
Segment Reporting Information
Premium Income	2,509.1	2,416.0	4,968.4	4,817.4
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,641.4	1,571.0	3,251.0	3,114.0
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	516.1	474.4	1,020.8	938.3
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	256.2	232.1	496.5	453.7
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	418.2	419.6	831.3	832.2
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	43.6	43.8	87.1	85.9
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	213.4	214.4	427.9	433.0
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	125.8	117.8	250.0	231.4
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	68.1	68.9	137.4	139.5
Operating Segments | Unum International
Segment Reporting Information
Premium Income	207.9	179.4	396.5	367.2
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	104.7	94.3	196.4	197.7
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	41.1	33.3	80.3	65.5
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	33.5	29.4	65.3	58.4
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	28.6	22.4	54.5	45.6
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	430.6	427.6	860.1	858.3
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	235.6	238.2	471.3	477.9
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	106.6	101.3	211.9	203.0
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	88.4	88.1	176.9	177.4
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	229.2	238.0	460.8	477.9
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	174.0	173.7	349.1	348.5
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 55.2	$ 64.3	$ 111.7	$ 129.4