Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information
Premium Income	$ 2,153.6	$ 2,089.4	$ 6,438.7	$ 6,258.5
Net Investment Income	609.0	611.4	1,831.9	1,841.1
Other Income	46.7	51.5	148.1	154.6
Operating Segments
Segment Reporting Information
Premium Income	2,153.6	2,089.4	6,438.7	6,258.5
Net Investment Income	609.0	611.4	1,831.9	1,841.1
Other Income	46.7	51.5	148.1	154.6
Operating Revenue	2,809.3	2,752.3	8,418.7	8,254.2
Operating Income (Loss)	357.0	321.1	1,050.4	980.7
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,360.9	1,315.0	4,079.0	3,915.7
Net Investment Income	201.0	207.3	609.1	622.4
Other Income	26.5	28.7	85.3	85.6
Operating Revenue	1,588.4	1,551.0	4,773.4	4,623.7
Operating Income (Loss)	258.4	231.0	745.3	674.1
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	131.5	127.3	379.6	407.4
Net Investment Income	28.5	28.5	88.3	89.3
Other Income	0.6	0.0	0.6	0.2
Operating Revenue	160.6	155.8	468.5	496.9
Operating Income (Loss)	26.5	28.2	82.0	98.7
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	378.7	354.1	1,129.3	1,056.7
Net Investment Income	36.0	36.1	107.8	105.7
Other Income	0.3	0.4	0.8	1.0
Operating Revenue	415.0	390.6	1,237.9	1,163.4
Operating Income (Loss)	81.7	79.0	245.9	234.3
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	282.5	293.0	850.8	878.7
Net Investment Income	337.2	334.1	1,012.5	1,008.6
Other Income	18.9	21.5	59.9	65.3
Operating Revenue	638.6	648.6	1,923.2	1,952.6
Operating Income (Loss)	26.6	28.6	90.8	94.9
Operating Segments | Corporate and Other
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	6.3	5.4	14.2	15.1
Other Income	0.4	0.9	1.5	2.5
Operating Revenue	6.7	6.3	15.7	17.6
Operating Income (Loss)	$ (36.2)	$ (45.7)	$ (113.6)	$ (121.3)