Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information
Premium Income	$ 2,251.8	$ 2,153.6	$ 6,722.8	$ 6,438.7
Net Investment Income	619.2	609.0	1,845.1	1,831.9
Other Income	50.1	46.7	147.9	148.1
Operating Segments
Segment Reporting Information
Premium Income	2,251.8	2,153.6	6,722.8	6,438.7
Net Investment Income	619.2	609.0	1,845.1	1,831.9
Other Income	50.1	46.7	147.9	148.1
Adjusted Operating Revenue	2,921.1	2,809.3	8,715.8	8,418.7
Adjusted Operating Income (Loss)	366.3	357.0	1,067.0	1,050.4
Operating Segments \| Unum US
Segment Reporting Information
Premium Income	1,446.2	1,360.9	4,301.3	4,079.0
Net Investment Income	200.3	201.0	591.0	609.1
Other Income	30.7	26.5	88.5	85.3
Adjusted Operating Revenue	1,677.2	1,588.4	4,980.8	4,773.4
Adjusted Operating Income (Loss)	270.9	258.4	765.9	745.3
Operating Segments \| Unum UK
Segment Reporting Information
Premium Income	138.0	131.5	416.5	379.6
Net Investment Income	26.4	28.5	86.1	88.3
Other Income	0.0	0.6	0.0	0.6
Adjusted Operating Revenue	164.4	160.6	502.6	468.5
Adjusted Operating Income (Loss)	26.1	26.5	83.5	82.0
Operating Segments \| Colonial Life
Segment Reporting Information
Premium Income	400.0	378.7	1,193.7	1,129.3
Net Investment Income	36.7	36.0	114.2	107.8
Other Income	0.4	0.3	1.0	0.8
Adjusted Operating Revenue	437.1	415.0	1,308.9	1,237.9
Adjusted Operating Income (Loss)	84.2	81.7	249.8	245.9
Operating Segments \| Closed Block
Segment Reporting Information
Premium Income	267.6	282.5	811.3	850.8
Net Investment Income	348.0	337.2	1,031.3	1,012.5
Other Income	18.9	18.9	56.8	59.9
Adjusted Operating Revenue	634.5	638.6	1,899.4	1,923.2
Adjusted Operating Income (Loss)	32.2	26.6	90.7	90.8
Operating Segments \| Corporate and Other
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	7.8	6.3	22.5	14.2
Other Income	0.1	0.4	1.6	1.5
Adjusted Operating Revenue	7.9	6.7	24.1	15.7
Adjusted Operating Income (Loss)	$ (47.1)	$ (36.2)	$ (122.9)	$ (113.6)

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