Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information
Premium Income	$ 2,020.8	$ 1,947.2	$ 6,044.6	$ 5,829.3
Operating Segments
Segment Reporting Information
Premium Income	2,020.8	1,947.2	6,044.6	5,829.3
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,241.8	1,162.7	3,708.4	3,472.9
Operating Segments | Unum US | Group Long term Disability
Segment Reporting Information
Premium Income	414.3	387.7	1,229.0	1,157.4
Operating Segments | Unum US | Group Short term Disability
Segment Reporting Information
Premium Income	153.2	138.3	452.5	413.4
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	336.5	316.1	1,007.5	939.2
Operating Segments | Unum US | Accidental Death Dismemberment
Segment Reporting Information
Premium Income	32.1	32.2	98.0	93.9
Operating Segments | Unum US | Individual Disability Recently Issued
Segment Reporting Information
Premium Income	119.6	116.9	355.9	348.8
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	186.1	171.5	565.5	520.2
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	144.6	152.6	425.8	458.3
Operating Segments | Unum UK | Group Long term Disability
Segment Reporting Information
Premium Income	100.6	104.5	296.5	313.4
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income	29.7	34.1	90.6	102.6
Operating Segments | Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	14.3	14.0	38.7	42.3
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	333.1	319.4	1,001.2	952.6
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	196.5	190.7	591.1	569.0
Operating Segments | Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income	62.5	57.9	188.1	172.8
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	74.1	70.8	222.0	210.8
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	301.3	312.5	909.2	945.5
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	142.2	154.0	433.0	472.9
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	158.6	158.3	475.0	471.9
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 0.5	$ 0.2	$ 1.2	$ 0.7