Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information
Premium Income	$ 2,158.4	$ 2,153.6	$ 2,142.2	$ 2,142.9	$ 2,099.2	$ 2,089.4	$ 2,081.6	$ 2,087.5	$ 8,597.1	$ 8,357.7	$ 8,082.4
Accident, Sickness, and Disability
Segment Reporting Information
Premium Income									6,449.9	6,296.7	6,099.2
Life Insurance
Segment Reporting Information
Premium Income									2,147.2	2,061.0	1,983.2
Operating Segments
Segment Reporting Information
Premium Income									8,597.1	8,357.7	8,082.4
Operating Segments | Unum US
Segment Reporting Information
Premium Income									5,443.5	5,240.9	4,960.0
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income									1,749.6	1,726.6	1,644.7
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income									639.8	626.1	607.4
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income									1,467.5	1,410.0	1,347.4
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income									147.5	140.3	131.7
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income									420.2	480.3	478.9
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income									849.4	796.5	749.9
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income									169.5	61.1	0.0
Operating Segments | Unum UK
Segment Reporting Information
Premium Income									513.0	529.3	576.2
Operating Segments | Unum UK | Group Long-term Disability
Segment Reporting Information
Premium Income									340.3	355.2	397.4
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income									103.1	105.7	121.5
Operating Segments | Unum UK | Supplemental
Segment Reporting Information
Premium Income									69.6	68.4	57.3
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income									1,511.4	1,417.1	1,338.6
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income									884.2	830.0	789.0
Operating Segments | Colonial Life | Life Insurance
Segment Reporting Information
Premium Income									300.4	273.8	252.4
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income									326.8	313.3	297.2
Operating Segments | Closed Block
Segment Reporting Information
Premium Income									1,129.2	1,170.4	1,207.6
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income									471.8	521.9	572.4
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income									648.7	643.9	633.5
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income									$ 8.7	$ 4.6	$ 1.7