Selected Operating Statement Data by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Segment Reporting Information [Line Items]
Premium Income	$ 1,938.5	$ 1,930.9
Net Investment Income	612.5	621.1
Other Income	53.8	62.5
Operating Revenue	2,604.8	2,614.5
Operating Income (Loss)	321.6	308.4
Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	1,152.5	1,139.7
Net Investment Income	226.4	235.7
Other Income	30.9	36.7
Operating Revenue	1,409.8	1,412.1
Operating Income (Loss)	210.8	208.1
Unum U.K.
Segment Reporting Information [Line Items]
Premium Income	151.7	143.8
Net Investment Income	33.6	30.8
Other Income	(0.1)	0.1
Operating Revenue	185.2	174.7
Operating Income (Loss)	36.5	31.3
Colonial Life
Segment Reporting Information [Line Items]
Premium Income	316.4	307.1
Net Investment Income	36.9	39.7
Other Income	(0.1)	0
Operating Revenue	353.2	346.8
Operating Income (Loss)	79.4	75.4
Closed Block
Segment Reporting Information [Line Items]
Premium Income	317.9	340.3
Net Investment Income	316.3	313.1
Other Income	21.6	24.1
Operating Revenue	655.8	677.5
Operating Income (Loss)	29.1	27.3
Corporate
Segment Reporting Information [Line Items]
Premium Income	0	0
Net Investment Income	(0.7)	1.8
Other Income	1.5	1.6
Operating Revenue	0.8	3.4
Operating Income (Loss)	$ (34.2)	$ (33.7)