Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information
Premium Income	$ 2,221.0	$ 2,142.2	$ 4,471.0	$ 4,285.1
Operating Segments
Segment Reporting Information
Premium Income	2,221.0	2,142.2	4,471.0	4,285.1
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,416.3	1,357.7	2,855.1	2,718.1
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	437.4	437.7	878.6	872.1
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	171.0	158.3	347.3	316.2
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	391.1	363.9	790.3	731.7
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	38.6	36.7	77.3	73.3
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	104.7	106.7	209.6	213.7
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	223.9	213.2	453.7	428.4
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	49.6	41.2	98.3	82.7
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	138.9	126.8	278.5	248.1
Operating Segments | Unum UK | Group Long-term Disability
Segment Reporting Information
Premium Income	89.7	83.8	180.5	164.6
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income	27.9	25.4	56.5	50.2
Operating Segments | Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	21.3	17.6	41.5	33.3
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	395.4	376.3	793.7	750.6
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	228.6	220.1	459.9	439.2
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	81.5	75.0	162.5	149.2
Operating Segments | Colonial Life | Cancer and Critical Illness
Segment Reporting Information
Premium Income	85.3	81.2	171.3	162.2
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	270.4	281.4	543.7	568.3
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	106.9	118.5	216.3	239.8
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	161.5	160.9	322.8	324.0
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 2.0	$ 2.0	$ 4.6	$ 4.5