Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information
Premium Income	$ 2,251.8	$ 2,153.6	$ 6,722.8	$ 6,438.7
Operating Segments
Segment Reporting Information
Premium Income	2,251.8	2,153.6	6,722.8	6,438.7
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,446.2	1,360.9	4,301.3	4,079.0
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	444.1	435.2	1,322.7	1,307.3
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	180.4	161.1	527.7	477.3
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	399.3	370.0	1,189.6	1,101.7
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	39.6	37.2	116.9	110.5
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	109.5	104.4	319.1	318.1
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	222.3	210.8	676.0	639.2
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	51.0	42.2	149.3	124.9
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	138.0	131.5	416.5	379.6
Operating Segments | Unum UK | Group Long-term Disability
Segment Reporting Information
Premium Income	91.3	87.0	271.8	251.6
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income	27.1	26.7	83.6	76.9
Operating Segments | Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	19.6	17.8	61.1	51.1
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	400.0	378.7	1,193.7	1,129.3
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	231.9	222.3	691.8	661.5
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	81.4	74.6	243.9	223.8
Operating Segments | Colonial Life | Cancer and Critical Illness
Segment Reporting Information
Premium Income	86.7	81.8	258.0	244.0
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	267.6	282.5	811.3	850.8
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	102.7	117.9	319.0	357.7
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	163.0	162.4	485.8	486.4
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 1.9	$ 2.2	$ 6.5	$ 6.7