Selected Operating Statement Data by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting Information [Line Items]
Premium Income	$ 1,921.9	$ 1,869.5
Net Investment Income	619.5	618.7
Other Income	57.5	59.7
Operating Revenue	2,598.9	2,547.9
Operating Income (Loss)	309.2	320.2
Interest and Debt Expense	35.5	37.9
Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	1,112.0	1,068.8
Net Investment Income	238.6	233.7
Other Income	31.1	31.0
Operating Revenue	1,381.7	1,333.5
Operating Income (Loss)	205.9	194.7
Unum U.K.
Segment Reporting Information [Line Items]
Premium Income	170.7	167.1
Net Investment Income	36.5	44.9
Other Income	0.1	0.1
Operating Revenue	207.3	212.1
Operating Income (Loss)	38.8	48.9
Colonial Life
Segment Reporting Information [Line Items]
Premium Income	296.3	280.4
Net Investment Income	35.6	31.3
Other Income	0.1	0.1
Operating Revenue	332.0	311.8
Operating Income (Loss)	69.7	66.5
Closed Block
Segment Reporting Information [Line Items]
Premium Income	342.9	353.2
Net Investment Income	301.6	294.3
Other Income	25.9	27.5
Operating Revenue	670.4	675.0
Operating Income (Loss)	15.4	31.9
Corporate
Segment Reporting Information [Line Items]
Premium Income	0	0
Net Investment Income	7.2	14.5
Other Income	0.3	1.0
Operating Revenue	7.5	15.5
Operating Income (Loss)	$ (20.6)	$ (21.8)