Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information
Premium Income	$ 2,263.3	$ 2,251.8	$ 2,221.0	$ 2,250.0	$ 2,158.4	$ 2,153.6	$ 2,142.2	$ 2,142.9	$ 8,986.1	$ 8,597.1	$ 8,357.7
Accident, Sickness, and Disability
Segment Reporting Information
Premium Income									6,673.5	6,449.9	6,296.7
Life Insurance
Segment Reporting Information
Premium Income									2,312.6	2,147.2	2,061.0
Operating Segments
Segment Reporting Information
Premium Income									8,986.1	8,597.1	8,357.7
Operating Segments | Unum US
Segment Reporting Information
Premium Income									5,736.4	5,443.5	5,240.9
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income									1,766.2	1,749.6	1,726.6
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income									706.3	639.8	626.1
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income									1,583.7	1,467.5	1,410.0
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income									156.3	147.5	140.3
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income									425.4	420.2	480.3
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income									895.7	849.4	796.5
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income									202.8	169.5	61.1
Operating Segments | Unum International
Segment Reporting Information
Premium Income									568.8	513.0	529.3
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income									358.9	340.3	355.2
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income									110.8	103.1	105.7
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income									99.1	69.6	68.4
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income									1,603.8	1,511.4	1,417.1
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income									929.3	884.2	830.0
Operating Segments | Colonial Life | Life Insurance
Segment Reporting Information
Premium Income									328.4	300.4	273.8
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income									346.1	326.8	313.3
Operating Segments | Closed Block
Segment Reporting Information
Premium Income									1,077.1	1,129.2	1,170.4
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income									420.8	471.8	521.9
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income									648.3	648.7	643.9
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income									$ 8.0	$ 8.7	$ 4.6