Selected Operating Statement Data by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Premium Income	$ 1,897.3	$ 1,929.4	$ 5,734.0	$ 5,778.9
Net Investment Income	615.5	619.2	1,862.7	1,872.2
Other Income	54.2	58.1	173.4	174.5
Operating Revenue	2,567.0	2,606.7	7,770.1	7,825.6
Operating Income (Loss)	312.9	310.7	932.4	930.0
Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	1,124.6	1,111.9	3,395.8	3,338.9
Net Investment Income	233.1	236.3	699.9	711.8
Other Income	30.2	31.3	99.0	93.1
Operating Revenue	1,387.9	1,379.5	4,194.7	4,143.8
Operating Income (Loss)	219.8	216.3	641.9	634.9
Unum U.K.
Segment Reporting Information [Line Items]
Premium Income	137.3	175.2	418.7	519.1
Net Investment Income	30.3	34.8	106.5	120.5
Other Income	0.1	0	0.1	0.1
Operating Revenue	167.7	210.0	525.3	639.7
Operating Income (Loss)	31.3	27.5	96.1	96.3
Colonial Life
Segment Reporting Information [Line Items]
Premium Income	309.1	299.4	924.1	892.6
Net Investment Income	36.3	33.9	110.3	103.5
Other Income	0	0	0.1	0.2
Operating Revenue	345.4	333.3	1,034.5	996.3
Operating Income (Loss)	69.0	68.7	215.5	206.0
Closed Block
Segment Reporting Information [Line Items]
Premium Income	326.3	342.9	995.4	1,028.3
Net Investment Income	315.4	307.3	944.6	913.4
Other Income	23.6	26.4	71.5	78.9
Operating Revenue	665.3	676.6	2,011.5	2,020.6
Operating Income (Loss)	25.7	25.6	82.6	66.7
Corporate
Segment Reporting Information [Line Items]
Premium Income	0	0	0	0
Net Investment Income	0.4	6.9	1.4	23.0
Other Income	0.3	0.4	2.7	2.2
Operating Revenue	0.7	7.3	4.1	25.2
Operating Income (Loss)	$ (32.9)	$ (27.4)	$ (103.7)	$ (73.9)