Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information
Premium Income	$ 2,352.6	$ 2,331.2	$ 2,343.1	$ 2,338.7	$ 2,263.3	$ 2,251.8	$ 2,221.0	$ 2,250.0	$ 9,365.6	$ 8,986.1	$ 8,597.1
Accident, Sickness, and Disability
Segment Reporting Information
Premium Income									6,944.9	6,673.5	6,449.9
Life Insurance
Segment Reporting Information
Premium Income									2,420.7	2,312.6	2,147.2
Operating Segments
Segment Reporting Information
Premium Income									9,365.6	8,986.1	8,597.1
Operating Segments | Unum US
Segment Reporting Information
Premium Income									6,016.6	5,736.4	5,443.5
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income									1,823.1	1,766.2	1,749.6
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income									768.8	706.3	639.8
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income									1,662.0	1,583.7	1,467.5
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income									165.7	156.3	147.5
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income									440.7	425.4	420.2
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income									910.2	895.7	849.4
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income									246.1	202.8	169.5
Operating Segments | Unum International
Segment Reporting Information
Premium Income									630.5	568.8	513.0
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income									353.4	358.9	340.3
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income									115.7	110.8	103.1
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income									89.5	81.7	69.6
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income									71.9	17.4	0.0
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income									1,685.0	1,603.8	1,511.4
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income									973.4	929.3	884.2
Operating Segments | Colonial Life | Life Insurance
Segment Reporting Information
Premium Income									351.6	328.4	300.4
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income									360.0	346.1	326.8
Operating Segments | Closed Block
Segment Reporting Information
Premium Income									1,033.5	1,077.1	1,129.2
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income									374.3	420.8	471.8
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income									651.6	648.3	648.7
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income									$ 7.6	$ 8.0	$ 8.7