Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information
Premium Income	$ 2,250.0	$ 2,142.9
Net Investment Income	602.3	602.4
Other Income	49.5	50.2
Operating Segments
Segment Reporting Information
Premium Income	2,250.0	2,142.9
Net Investment Income	602.3	602.4
Other Income	49.5	50.2
Adjusted Operating Revenue	2,901.8	2,795.5
Adjusted Operating Income (Loss)	343.3	339.9
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,438.8	1,360.4
Net Investment Income	194.2	202.5
Other Income	29.0	28.7
Adjusted Operating Revenue	1,662.0	1,591.6
Adjusted Operating Income (Loss)	243.9	239.1
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	139.6	121.3
Net Investment Income	27.6	26.6
Other Income	0.0	0.0
Adjusted Operating Revenue	167.2	147.9
Adjusted Operating Income (Loss)	29.8	26.6
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	398.3	374.3
Net Investment Income	37.3	35.1
Other Income	0.3	0.3
Adjusted Operating Revenue	435.9	409.7
Adjusted Operating Income (Loss)	81.0	82.4
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	273.3	286.9
Net Investment Income	337.7	335.3
Other Income	19.0	20.8
Adjusted Operating Revenue	630.0	643.0
Adjusted Operating Income (Loss)	28.9	31.6
Operating Segments | Corporate and Other
Segment Reporting Information
Premium Income	0.0	0.0
Net Investment Income	5.5	2.9
Other Income	1.2	0.4
Adjusted Operating Revenue	6.7	3.3
Adjusted Operating Income (Loss)	$ (40.3)	$ (39.8)