Selected Operating Statement Data by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Premium Income	$ 1,930.9	$ 1,921.9
Net Investment Income	621.1	619.5
Other Income	62.5	57.5
Operating Revenue	2,614.5	2,598.9
Operating Income (Loss)	308.4	309.2
Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	1,139.7	1,112.0
Net Investment Income	235.7	238.6
Other Income	36.7	31.1
Operating Revenue	1,412.1	1,381.7
Operating Income (Loss)	208.1	205.9
Unum U.K.
Segment Reporting Information [Line Items]
Premium Income	143.8	170.7
Net Investment Income	30.8	36.5
Other Income	0.1	0.1
Operating Revenue	174.7	207.3
Operating Income (Loss)	31.3	38.8
Colonial Life
Segment Reporting Information [Line Items]
Premium Income	307.1	296.3
Net Investment Income	39.7	35.6
Other Income	0	0.1
Operating Revenue	346.8	332.0
Operating Income (Loss)	75.4	69.7
Closed Block
Segment Reporting Information [Line Items]
Premium Income	340.3	342.9
Net Investment Income	313.1	301.6
Other Income	24.1	25.9
Operating Revenue	677.5	670.4
Operating Income (Loss)	27.3	15.4
Corporate
Segment Reporting Information [Line Items]
Net Investment Income	1.8	7.2
Other Income	1.6	0.3
Operating Revenue	3.4	7.5
Operating Income (Loss)	$ (33.7)	$ (20.6)