Selected Operating Statement Data by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information
Premium Income	$ 1,947.2	$ 1,897.3	$ 5,829.3	$ 5,734.0
Net Investment Income	606.4	615.5	1,848.0	1,862.7
Other Income	54.6	54.2	163.3	173.4
Operating Segments
Segment Reporting Information
Premium Income	1,947.2	1,897.3	5,829.3	5,734.0
Net Investment Income	606.4	615.5	1,848.0	1,862.7
Other Income	54.6	54.2	163.3	173.4
Operating Revenue	2,608.2	2,567.0	7,840.6	7,770.1
Operating Income (Loss)	312.3	312.9	969.1	932.4
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,162.7	1,124.6	3,472.9	3,395.8
Net Investment Income	216.6	233.1	664.2	699.9
Other Income	30.2	30.2	91.4	99.0
Operating Revenue	1,409.5	1,387.9	4,228.5	4,194.7
Operating Income (Loss)	214.3	219.8	643.9	641.9
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	152.6	137.3	458.3	418.7
Net Investment Income	34.4	30.3	112.3	106.5
Other Income	0	0.1	0.1	0.1
Operating Revenue	187.0	167.7	570.7	525.3
Operating Income (Loss)	33.5	31.3	109.6	96.1
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	319.4	309.1	952.6	924.1
Net Investment Income	35.7	36.3	108.8	110.3
Other Income	0.1	0	0.1	0.1
Operating Revenue	355.2	345.4	1,061.5	1,034.5
Operating Income (Loss)	71.2	69.0	225.9	215.5
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	312.5	326.3	945.5	995.4
Net Investment Income	317.7	315.4	960.5	944.6
Other Income	23.4	23.6	69.5	71.5
Operating Revenue	653.6	665.3	1,975.5	2,011.5
Operating Income (Loss)	26.2	25.7	92.6	82.6
Operating Segments | Corporate and Other
Segment Reporting Information
Premium Income	0	0	0	0
Net Investment Income	2.0	0.4	2.2	1.4
Other Income	0.9	0.3	2.2	2.7
Operating Revenue	2.9	0.7	4.4	4.1
Operating Income (Loss)	$ (32.9)	$ (32.9)	$ (102.9)	$ (103.7)