Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information
Premium Income	$ 2,099.2	$ 2,089.4	$ 2,081.6	$ 2,087.5	$ 2,037.8	$ 2,020.8	$ 2,017.5	$ 2,006.3	$ 8,357.7	$ 8,082.4	$ 7,797.2
Accident, Sickness, and Disability
Segment Reporting Information
Premium Income									6,296.7	6,099.2	5,926.0
Life Insurance
Segment Reporting Information
Premium Income									2,061.0	1,983.2	1,871.2
Operating Segments
Segment Reporting Information
Premium Income									8,357.7	8,082.4	7,797.2
Operating Segments | Unum US
Segment Reporting Information
Premium Income									5,240.9	4,960.0	4,659.7
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income									1,726.6	1,644.7	1,553.5
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income									626.1	607.4	558.1
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income									1,410.0	1,347.4	1,262.3
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income									140.3	131.7	125.9
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income									480.3	478.9	466.1
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income									796.5	749.9	693.8
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income									61.1	0.0	0.0
Operating Segments | Unum UK
Segment Reporting Information
Premium Income									529.3	576.2	607.2
Operating Segments | Unum UK | Group Long-term Disability
Segment Reporting Information
Premium Income									355.2	397.4	418.9
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income									105.7	121.5	133.2
Operating Segments | Unum UK | Supplemental
Segment Reporting Information
Premium Income									68.4	57.3	55.1
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income									1,417.1	1,338.6	1,273.7
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income									830.0	789.0	759.8
Operating Segments | Colonial Life | Life Insurance
Segment Reporting Information
Premium Income									273.8	252.4	231.8
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income									313.3	297.2	282.1
Operating Segments | Closed Block
Segment Reporting Information
Premium Income									1,170.4	1,207.6	1,256.6
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income									521.9	572.4	624.8
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income									643.9	633.5	630.9
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income									$ 4.6	$ 1.7	$ 0.9