Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information
Premium Income	$ 2,153.6	$ 2,089.4	$ 6,438.7	$ 6,258.5
Operating Segments
Segment Reporting Information
Premium Income	2,153.6	2,089.4	6,438.7	6,258.5
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,360.9	1,315.0	4,079.0	3,915.7
Operating Segments | Unum US | Group Long term Disability
Segment Reporting Information
Premium Income	435.2	428.1	1,307.3	1,291.6
Operating Segments | Unum US | Group Short term Disability
Segment Reporting Information
Premium Income	161.1	155.5	477.3	469.2
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	370.0	353.4	1,101.7	1,053.6
Operating Segments | Unum US | Accidental Death Dismemberment
Segment Reporting Information
Premium Income	37.2	35.3	110.5	104.8
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	104.4	125.6	318.1	374.4
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	210.8	197.1	639.2	602.1
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	42.2	20.0	124.9	20.0
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	131.5	127.3	379.6	407.4
Operating Segments | Unum UK | Group Long term Disability
Segment Reporting Information
Premium Income	87.0	85.5	251.6	274.1
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income	26.7	24.9	76.9	80.9
Operating Segments | Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	17.8	16.9	51.1	52.4
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	378.7	354.1	1,129.3	1,056.7
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	222.3	207.7	661.5	619.2
Operating Segments | Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income	74.6	68.0	223.8	203.4
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	81.8	78.4	244.0	234.1
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	282.5	293.0	850.8	878.7
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	117.9	129.9	357.7	394.0
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	162.4	161.4	486.4	482.0
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 2.2	$ 1.7	$ 6.7	$ 2.7