Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information
Premium Income	$ 2,142.2	$ 2,081.6	$ 4,285.1	$ 4,169.1
Net Investment Income	620.5	623.3	1,222.9	1,229.7
Other Income	51.2	51.1	101.4	103.1
Operating Segments
Segment Reporting Information
Premium Income	2,142.2	2,081.6	4,285.1	4,169.1
Net Investment Income	620.5	623.3	1,222.9	1,229.7
Other Income	51.2	51.1	101.4	103.1
Operating Revenue	2,813.9	2,756.0	5,609.4	5,501.9
Operating Income (Loss)	353.5	334.9	693.4	659.6
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,357.7	1,299.4	2,718.1	2,600.7
Net Investment Income	205.6	207.7	408.1	415.1
Other Income	30.1	27.9	58.8	56.9
Operating Revenue	1,593.4	1,535.0	3,185.0	3,072.7
Operating Income (Loss)	247.8	227.2	486.9	443.1
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	126.8	140.8	248.1	280.1
Net Investment Income	33.2	34.0	59.8	60.8
Other Income	0.0	0.2	0.0	0.2
Operating Revenue	160.0	175.0	307.9	341.1
Operating Income (Loss)	28.9	36.9	55.5	70.5
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	376.3	351.4	750.6	702.6
Net Investment Income	36.7	35.6	71.8	69.6
Other Income	0.2	0.3	0.5	0.6
Operating Revenue	413.2	387.3	822.9	772.8
Operating Income (Loss)	81.8	77.9	164.2	155.3
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	281.4	290.0	568.3	585.7
Net Investment Income	340.0	341.1	675.3	674.5
Other Income	20.2	21.4	41.0	43.8
Operating Revenue	641.6	652.5	1,284.6	1,304.0
Operating Income (Loss)	32.6	32.6	64.2	66.3
Operating Segments | Corporate and Other
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	5.0	4.9	7.9	9.7
Other Income	0.7	1.3	1.1	1.6
Operating Revenue	5.7	6.2	9.0	11.3
Operating Income (Loss)	$ (37.6)	$ (39.7)	$ (77.4)	$ (75.6)