Premium Income by Major Line of Business within Each 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
Operating Segments
Segment Reporting Information
Premium Income	2,017.5	1,943.6	4,023.8	3,882.1
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,236.4	1,157.7	2,466.6	2,310.2
Operating Segments | Unum US | Group Long term Disability
Segment Reporting Information
Premium Income	409.4	386.2	814.7	769.7
Operating Segments | Unum US | Group Short term Disability
Segment Reporting Information
Premium Income	151.6	137.8	299.3	275.1
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	336.0	313.2	671.0	623.1
Operating Segments | Unum US | Accidental Death Dismemberment
Segment Reporting Information
Premium Income	33.3	31.1	65.9	61.7
Operating Segments | Unum US | Individual Disability Recently Issued
Segment Reporting Information
Premium Income	118.4	116.0	236.3	231.9
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	187.7	173.4	379.4	348.7
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	142.2	154.0	281.2	305.7
Operating Segments | Unum UK | Group Long term Disability
Segment Reporting Information
Premium Income	99.4	105.9	195.9	208.9
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income	30.4	33.7	60.9	68.5
Operating Segments | Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	12.4	14.4	24.4	28.3
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	335.9	316.8	668.1	633.2
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	198.1	189.2	394.6	378.3
Operating Segments | Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income	63.3	57.5	125.6	114.9
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	74.5	70.1	147.9	140.0
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	303.0	315.1	607.9	633.0
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	144.3	157.8	290.8	318.9
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	158.2	157.1	316.4	313.6
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 0.5	$ 0.2	$ 0.7	$ 0.5