Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information
Premium Income	$ 2,089.4	$ 2,020.8	$ 6,258.5	$ 6,044.6
Operating Segments
Segment Reporting Information
Premium Income	2,089.4	2,020.8	6,258.5	6,044.6
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,315.0	1,241.8	3,915.7	3,708.4
Operating Segments | Unum US | Group Long term Disability
Segment Reporting Information
Premium Income	428.1	414.3	1,291.6	1,229.0
Operating Segments | Unum US | Group Short term Disability
Segment Reporting Information
Premium Income	155.5	153.2	469.2	452.5
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	353.4	336.5	1,053.6	1,007.5
Operating Segments | Unum US | Accidental Death Dismemberment
Segment Reporting Information
Premium Income	35.3	32.1	104.8	98.0
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	125.6	119.6	374.4	355.9
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	197.1	186.1	602.1	565.5
Operating Segments | Unum US | Dental and Vision [Member]
Segment Reporting Information
Premium Income	20.0	0.0	20.0	0.0
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	127.3	144.6	407.4	425.8
Operating Segments | Unum UK | Group Long term Disability
Segment Reporting Information
Premium Income	85.5	100.6	274.1	296.5
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income	24.9	29.7	80.9	90.6
Operating Segments | Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	16.9	14.3	52.4	38.7
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	354.1	333.1	1,056.7	1,001.2
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	207.7	196.5	619.2	591.1
Operating Segments | Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income	68.0	62.5	203.4	188.1
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	78.4	74.1	234.1	222.0
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	293.0	301.3	878.7	909.2
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	129.9	142.2	394.0	433.0
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	161.4	158.6	482.0	475.0
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 1.7	$ 0.5	$ 2.7	$ 1.2