Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information
Premium Income	$ 2,020.8	$ 1,947.2	$ 6,044.6	$ 5,829.3
Net Investment Income	612.1	611.3	1,844.8	1,859.5
Other Income	51.5	54.6	160.6	163.3
Operating Segments
Segment Reporting Information
Premium Income	2,020.8	1,947.2	6,044.6	5,829.3
Net Investment Income	612.1	611.3	1,844.8	1,859.5
Other Income	51.5	54.6	160.6	163.3
Operating Revenue	2,684.4	2,613.1	8,050.0	7,852.1
Operating Income (Loss)	328.5	317.2	971.7	980.6
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,241.8	1,162.7	3,708.4	3,472.9
Net Investment Income	214.3	214.4	645.0	655.7
Other Income	28.1	30.2	90.4	91.4
Operating Revenue	1,484.2	1,407.3	4,443.8	4,220.0
Operating Income (Loss)	218.7	212.1	635.8	635.4
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	144.6	152.6	425.8	458.3
Net Investment Income	28.0	34.4	90.1	112.3
Other Income	0.0	0.0	0.0	0.1
Operating Revenue	172.6	187.0	515.9	570.7
Operating Income (Loss)	32.7	33.5	103.6	109.6
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	333.1	319.4	1,001.2	952.6
Net Investment Income	35.9	35.5	109.8	107.9
Other Income	0.1	0.1	0.1	0.1
Operating Revenue	369.1	355.0	1,111.1	1,060.6
Operating Income (Loss)	76.3	71.0	231.5	225.0
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	301.3	312.5	909.2	945.5
Net Investment Income	327.5	317.2	979.6	958.5
Other Income	21.5	23.4	67.5	69.5
Operating Revenue	650.3	653.1	1,956.3	1,973.5
Operating Income (Loss)	27.7	25.7	91.0	90.6
Operating Segments | Corporate and Other
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	6.4	9.8	20.3	25.1
Other Income	1.8	0.9	2.6	2.2
Operating Revenue	8.2	10.7	22.9	27.3
Operating Income (Loss)	$ (26.9)	$ (25.1)	$ (90.2)	$ (80.0)