Selected Operating Statement Data by Segment (Detail) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Premium Income	$ 1,881.2	$ 1,850.2	$ 5,625.7	$ 5,563.2
Net Investment Income	629.2	618.4	1,885.0	1,861.2
Other Income	59.1	58.2	174.8	178.5
Operating Revenue	2,569.5	2,526.8	7,685.5	7,602.9
Operating Income (Loss)	317.2	325.4	973.8	998.6
Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	1,226.5	1,210.6	3,666.5	3,643.2
Net Investment Income	332.4	312.9	983.1	936.2
Other Income	30.4	29.7	91.1	90.6
Operating Revenue	1,589.3	1,553.2	4,740.7	4,670.0
Operating Income (Loss)	219.5	204.7	646.7	619.9
Unum U.K.
Segment Reporting Information [Line Items]
Premium Income	175.5	161.4	518.4	480.1
Net Investment Income	43.3	39.0	139.3	121.9
Other Income	0	0.2	0.1	1.1
Operating Revenue	218.8	200.6	657.8	603.1
Operating Income (Loss)	34.9	47.2	138.3	160.7
Colonial Life
Segment Reporting Information [Line Items]
Premium Income	283.7	269.3	846.1	802.1
Net Investment Income	32.5	31.9	99.6	91.1
Other Income	0.1	0.2	0.4	0.5
Operating Revenue	316.3	301.4	946.1	893.7
Operating Income (Loss)	70.3	74.5	214.5	221.4
Individual Disability-Closed Block
Segment Reporting Information [Line Items]
Premium Income	196.4	208.5	595.0	635.0
Net Investment Income	181.9	185.5	544.2	563.5
Other Income	23.6	22.7	67.9	67.3
Operating Revenue	401.9	416.7	1,207.1	1,265.8
Operating Income (Loss)	14.0	9.8	34.3	33.9
Corporate and Other
Segment Reporting Information [Line Items]
Premium Income	(0.9)	0.4	(0.3)	2.8
Net Investment Income	39.1	49.1	118.8	148.5
Other Income	5.0	5.4	15.3	19.0
Operating Revenue	43.2	54.9	133.8	170.3
Operating Income (Loss)	$ (21.5)	$ (10.8)	$ (60.0)	$ (37.3)