Premium Income by Major Line of Business within Each Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information
Premium Income	$ 2,006.3	$ 1,938.5
Operating Segments
Segment Reporting Information
Premium Income	2,006.3	1,938.5
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,230.2	1,152.5
Operating Segments | Unum US | Group Long term Disability
Segment Reporting Information
Premium Income	405.3	383.5
Operating Segments | Unum US | Group Short term Disability
Segment Reporting Information
Premium Income	147.7	137.3
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	335.0	309.9
Operating Segments | Unum US | Accidental Death Dismemberment
Segment Reporting Information
Premium Income	32.6	30.6
Operating Segments | Unum US | Individual Disability Recently Issued
Segment Reporting Information
Premium Income	117.9	115.9
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	191.7	175.3
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	139.0	151.7
Operating Segments | Unum UK | Group Long term Disability
Segment Reporting Information
Premium Income	96.5	103.0
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income	30.5	34.8
Operating Segments | Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	12.0	13.9
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	332.2	316.4
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	196.5	189.1
Operating Segments | Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income	62.3	57.4
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	73.4	69.9
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	304.9	317.9
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	146.5	161.1
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	158.2	156.5
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 0.2	$ 0.3