Selected Operating Statement Data by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Premium Income	$ 1,929.4	$ 1,881.2	$ 5,778.9	$ 5,625.7
Net Investment Income	619.2	629.2	1,872.2	1,885.0
Other Income	58.1	59.1	174.5	174.8
Operating Revenue	2,606.7	2,569.5	7,825.6	7,685.5
Operating Income (Loss)	310.7	320.6	930.0	986.9
Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	1,111.9	1,074.4	3,338.9	3,213.0
Net Investment Income	236.3	240.0	711.8	712.6
Other Income	31.3	30.4	93.1	90.9
Operating Revenue	1,379.5	1,344.8	4,143.8	4,016.5
Operating Income (Loss)	216.3	209.2	634.9	609.5
Unum U.K.
Segment Reporting Information [Line Items]
Premium Income	175.2	175.5	519.1	518.4
Net Investment Income	34.8	43.3	120.5	139.3
Other Income	0	0	0.1	0.1
Operating Revenue	210.0	218.8	639.7	657.8
Operating Income (Loss)	27.5	34.7	96.3	138.8
Colonial Life
Segment Reporting Information [Line Items]
Premium Income	299.4	283.7	892.6	846.1
Net Investment Income	33.9	32.5	103.5	99.6
Other Income	0	0.1	0.2	0.4
Operating Revenue	333.3	316.3	996.3	946.1
Operating Income (Loss)	68.7	66.7	206.0	205.1
Closed Block
Segment Reporting Information [Line Items]
Premium Income	342.9	347.6	1,028.3	1,048.2
Net Investment Income	307.3	300.1	913.4	888.9
Other Income	26.4	26.4	78.9	80.6
Operating Revenue	676.6	674.1	2,020.6	2,017.7
Operating Income (Loss)	25.6	30.8	66.7	93.1
Corporate
Segment Reporting Information [Line Items]
Premium Income	0	0	0	0
Net Investment Income	6.9	13.3	23.0	44.6
Other Income	0.4	2.2	2.2	2.8
Operating Revenue	7.3	15.5	25.2	47.4
Operating Income (Loss)	$ (27.4)	$ (20.8)	$ (73.9)	$ (59.6)