Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information
Premium Income	$ 2,037.8	$ 2,020.8	$ 2,017.5	$ 2,006.3	$ 1,967.9	$ 1,947.2	$ 1,943.6	$ 1,938.5	$ 8,082.4	$ 7,797.2	$ 7,624.7
Net Investment Income	$ 636.4	$ 612.1	$ 630.7	$ 602.0	$ 632.7	$ 611.3	$ 632.4	$ 615.8	2,481.2	2,492.2	2,506.9
Other Income									211.5	219.0	230.2
Interest and Debt Expense									152.8	167.5	149.4
Operating Segments
Segment Reporting Information
Premium Income									8,082.4	7,797.2	7,624.7
Net Investment Income									2,481.2	2,492.2	2,506.9
Other Income									211.5	219.0	230.2
Operating Revenue									10,775.1	10,508.4	10,361.8
Operating Income (Loss)									1,294.0	1,307.3	1,256.6
Interest and Debt Expense									152.8	154.3	149.4
Depreciation and Amortization									587.1	534.4	509.8
Operating Segments | Unum U.S.
Segment Reporting Information
Premium Income									4,960.0	4,659.7	4,517.1
Net Investment Income									865.3	878.9	919.4
Other Income									119.2	122.1	128.3
Operating Revenue									5,944.5	5,660.7	5,564.8
Operating Income (Loss)									850.0	844.9	848.8
Interest and Debt Expense									0.0	0.0	0.1
Depreciation and Amortization									347.0	314.2	292.5
Operating Segments | Unum U.K.
Segment Reporting Information
Premium Income									576.2	607.2	556.6
Net Investment Income									124.9	151.0	148.5
Other Income									0.0	0.0	0.1
Operating Revenue									701.1	758.2	705.2
Operating Income (Loss)									140.6	147.8	132.0
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									18.0	19.2	22.5
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income									1,338.6	1,273.7	1,232.2
Net Investment Income									145.4	145.5	144.1
Other Income									0.1	0.1	0.2
Operating Revenue									1,484.1	1,419.3	1,376.5
Operating Income (Loss)									309.1	299.0	283.6
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									214.6	194.1	188.7
Operating Segments | Closed Block
Segment Reporting Information
Premium Income									1,207.6	1,256.6	1,318.8
Net Investment Income									1,320.0	1,281.5	1,270.2
Other Income									89.3	91.8	93.9
Operating Revenue									2,616.9	2,629.9	2,682.9
Operating Income (Loss)									119.1	120.0	107.3
Interest and Debt Expense									6.6	7.3	8.4
Depreciation and Amortization									6.6	5.9	5.2
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Premium Income									0.0	0.0	0.0
Net Investment Income									25.6	35.3	24.7
Other Income									2.9	5.0	7.7
Operating Revenue									28.5	40.3	32.4
Operating Income (Loss)									(124.8)	(104.4)	(115.1)
Interest and Debt Expense									146.2	147.0	140.9
Depreciation and Amortization									$ 0.9	$ 1.0	$ 0.9