Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information
Premium Income	$ 2,017.5	$ 1,943.6	$ 4,023.8	$ 3,882.1
Net Investment Income	630.7	632.4	1,232.7	1,248.2
Other Income	54.7	54.9	109.1	108.7
Operating Segments
Segment Reporting Information
Premium Income	2,017.5	1,943.6	4,023.8	3,882.1
Net Investment Income	630.7	632.4	1,232.7	1,248.2
Other Income	54.7	54.9	109.1	108.7
Operating Revenue	2,702.9	2,630.9	5,365.6	5,239.0
Operating Income (Loss)	321.9	338.5	643.2	663.4
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,236.4	1,157.7	2,466.6	2,310.2
Net Investment Income	215.7	218.2	430.7	441.3
Other Income	31.4	30.3	62.3	61.2
Operating Revenue	1,483.5	1,406.2	2,959.6	2,812.7
Operating Income (Loss)	202.8	215.8	417.1	423.3
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	142.2	154.0	281.2	305.7
Net Investment Income	38.6	44.3	62.1	77.9
Other Income	0.0	0.2	0.0	0.1
Operating Revenue	180.8	198.5	343.3	383.7
Operating Income (Loss)	38.3	39.6	70.9	76.1
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	335.9	316.8	668.1	633.2
Net Investment Income	36.9	35.8	73.9	72.4
Other Income	0.0	0.1	0.0	0.0
Operating Revenue	372.8	352.7	742.0	705.6
Operating Income (Loss)	77.6	74.9	155.2	154.0
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	303.0	315.1	607.9	633.0
Net Investment Income	331.7	325.8	652.1	641.3
Other Income	22.8	24.5	46.0	46.1
Operating Revenue	657.5	665.4	1,306.0	1,320.4
Operating Income (Loss)	36.6	36.6	63.3	64.9
Operating Segments | Corporate and Other
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	7.8	8.3	13.9	15.3
Other Income	0.5	(0.2)	0.8	1.3
Operating Revenue	8.3	8.1	14.7	16.6
Operating Income (Loss)	$ (33.4)	$ (28.4)	$ (63.3)	$ (54.9)