Premium Income by Major Line of Business within Each Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information
Premium Income	$ 1,947.2	$ 1,897.3	$ 5,829.3	$ 5,734.0
Operating Segments
Segment Reporting Information
Premium Income	1,947.2	1,897.3	5,829.3	5,734.0
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,162.7	1,124.6	3,472.9	3,395.8
Operating Segments | Unum US | Group Long term Disability
Segment Reporting Information
Premium Income	387.7	385.0	1,157.4	1,172.5
Operating Segments | Unum US | Group Short term Disability
Segment Reporting Information
Premium Income	138.3	129.5	413.4	389.6
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	316.1	305.2	939.2	911.0
Operating Segments | Unum US | Accidental Death Dismemberment
Segment Reporting Information
Premium Income	32.2	30.5	93.9	91.6
Operating Segments | Unum US | Individual Disability Recently Issued
Segment Reporting Information
Premium Income	116.9	114.9	348.8	349.3
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	171.5	159.5	520.2	481.8
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	152.6	137.3	458.3	418.7
Operating Segments | Unum UK | Group Long term Disability
Segment Reporting Information
Premium Income	104.5	96.5	313.4	290.7
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income	34.1	25.6	102.6	83.1
Operating Segments | Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	14.0	15.2	42.3	44.9
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	319.4	309.1	952.6	924.1
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	190.7	185.0	569.0	554.5
Operating Segments | Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income	57.9	55.6	172.8	165.7
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	70.8	68.5	210.8	203.9
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	312.5	326.3	945.5	995.4
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	154.0	169.3	472.9	521.7
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	158.3	157.0	471.9	473.6
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 0.2	$ 0	$ 0.7	$ 0.1