Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information
Premium Income	$ 2,089.4	$ 2,020.8	$ 6,258.5	$ 6,044.6
Net Investment Income	611.4	612.1	1,841.1	1,844.8
Other Income	51.5	51.5	154.6	160.6
Operating Segments
Segment Reporting Information
Premium Income	2,089.4	2,020.8	6,258.5	6,044.6
Net Investment Income	611.4	612.1	1,841.1	1,844.8
Other Income	51.5	51.5	154.6	160.6
Operating Revenue	2,752.3	2,684.4	8,254.2	8,050.0
Operating Income (Loss)	325.2	328.5	992.9	971.7
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,315.0	1,241.8	3,915.7	3,708.4
Net Investment Income	207.3	214.3	622.4	645.0
Other Income	28.7	28.1	85.6	90.4
Operating Revenue	1,551.0	1,484.2	4,623.7	4,443.8
Operating Income (Loss)	231.0	218.7	674.1	635.8
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	127.3	144.6	407.4	425.8
Net Investment Income	28.5	28.0	89.3	90.1
Other Income	0.0	0.0	0.2	0.0
Operating Revenue	155.8	172.6	496.9	515.9
Operating Income (Loss)	28.2	32.7	98.7	103.6
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	354.1	333.1	1,056.7	1,001.2
Net Investment Income	36.1	35.9	105.7	109.8
Other Income	0.4	0.1	1.0	0.1
Operating Revenue	390.6	369.1	1,163.4	1,111.1
Operating Income (Loss)	79.0	76.3	234.3	231.5
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	293.0	301.3	878.7	909.2
Net Investment Income	334.1	327.5	1,008.6	979.6
Other Income	21.5	21.5	65.3	67.5
Operating Revenue	648.6	650.3	1,952.6	1,956.3
Operating Income (Loss)	28.6	27.7	94.9	91.0
Operating Segments | Corporate and Other
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	5.4	6.4	15.1	20.3
Other Income	0.9	1.8	2.5	2.6
Operating Revenue	6.3	8.2	17.6	22.9
Operating Income (Loss)	$ (41.6)	$ (26.9)	$ (109.1)	$ (90.2)