Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information
Premium Income	$ 2,099.2	$ 2,089.4	$ 2,081.6	$ 2,087.5	$ 2,037.8	$ 2,020.8	$ 2,017.5	$ 2,006.3	$ 8,357.7	$ 8,082.4	$ 7,797.2
Net Investment Income	$ 617.9	$ 611.4	$ 623.3	$ 606.4	$ 636.4	$ 612.1	$ 630.7	$ 602.0	2,459.0	2,481.2	2,492.2
Other Income									205.6	211.5	219.0
Interest and Debt Expense									166.0	152.8	167.5
Operating Segments
Segment Reporting Information
Premium Income									8,357.7	8,082.4	7,797.2
Net Investment Income									2,459.0	2,481.2	2,492.2
Other Income									205.6	211.5	219.0
Operating Revenue									11,022.3	10,775.1	10,508.4
Operating Income (Loss)									1,339.8	1,294.0	1,307.3
Interest and Debt Expense									166.0	152.8	154.3
Depreciation and Amortization									600.2	587.1	534.4
Operating Segments | Unum US
Segment Reporting Information
Premium Income									5,240.9	4,960.0	4,659.7
Net Investment Income									828.7	865.3	878.9
Other Income									113.3	119.2	122.1
Operating Revenue									6,182.9	5,944.5	5,660.7
Operating Income (Loss)									914.2	850.0	844.9
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									353.0	347.0	314.2
Operating Segments | Unum UK
Segment Reporting Information
Premium Income									529.3	576.2	607.2
Net Investment Income									118.1	124.9	151.0
Other Income									0.2	0.0	0.0
Operating Revenue									647.6	701.1	758.2
Operating Income (Loss)									128.6	140.6	147.8
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									16.4	18.0	19.2
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income									1,417.1	1,338.6	1,273.7
Net Investment Income									141.5	145.4	145.5
Other Income									1.2	0.1	0.1
Operating Revenue									1,559.8	1,484.1	1,419.3
Operating Income (Loss)									314.2	309.1	299.0
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									222.9	214.6	194.1
Operating Segments | Closed Block
Segment Reporting Information
Premium Income									1,170.4	1,207.6	1,256.6
Net Investment Income									1,352.2	1,320.0	1,281.5
Other Income									86.0	89.3	91.8
Operating Revenue									2,608.6	2,616.9	2,629.9
Operating Income (Loss)									129.5	119.1	120.0
Interest and Debt Expense									6.9	6.6	7.3
Depreciation and Amortization									6.9	6.6	5.9
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Premium Income									0.0	0.0	0.0
Net Investment Income									18.5	25.6	35.3
Other Income									4.9	2.9	5.0
Operating Revenue									23.4	28.5	40.3
Operating Income (Loss)									(146.7)	(124.8)	(104.4)
Interest and Debt Expense									159.1	146.2	147.0
Depreciation and Amortization									$ 1.0	$ 0.9	$ 1.0