Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information
Premium Income	$ 2,319.9	$ 2,318.1	$ 2,368.7	$ 2,371.4	$ 2,352.6	$ 2,331.2	$ 2,343.1	$ 2,338.7	$ 9,378.1	$ 9,365.6	$ 8,986.1
Net Investment Income	$ 593.5	$ 613.2	$ 569.0	$ 585.0	$ 616.3	$ 599.4	$ 624.9	$ 594.7	2,360.7	2,435.3	2,453.7
Other Income									224.2	221.2	198.2
Interest and Debt Expense									188.2	177.4	167.3
Operating Segments
Segment Reporting Information
Premium Income									9,378.1	9,365.6	8,986.1
Net Investment Income									2,360.7	2,435.3	2,453.7
Other Income									224.2	221.2	198.2
Adjusted Operating Revenue									11,963.0	12,022.1	11,638.0
Adjusted Operating Income (Loss)									1,278.0	1,432.6	1,418.1
Interest and Debt Expense									188.2	177.4	167.3
Depreciation and Amortization									722.3	727.5	673.8
Operating Segments | Unum US
Segment Reporting Information
Premium Income									6,018.9	6,016.6	5,736.4
Net Investment Income									720.3	739.4	778.7
Other Income									154.9	142.8	118.5
Adjusted Operating Revenue									6,894.1	6,898.8	6,633.6
Adjusted Operating Income (Loss)									825.4	1,031.1	1,014.6
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									421.7	422.8	389.6
Operating Segments | Unum International
Segment Reporting Information
Premium Income									652.8	630.5	568.8
Net Investment Income									104.6	122.5	117.2
Other Income									0.5	0.6	0.4
Adjusted Operating Revenue									757.9	753.6	686.4
Adjusted Operating Income (Loss)									76.6	107.9	113.9
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									20.1	18.7	17.6
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income									1,712.0	1,685.0	1,603.8
Net Investment Income									155.7	148.0	151.2
Other Income									1.1	3.4	1.2
Adjusted Operating Revenue									1,868.8	1,836.4	1,756.2
Adjusted Operating Income (Loss)									335.4	344.5	335.2
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									273.9	276.6	257.3
Operating Segments | Closed Block
Segment Reporting Information
Premium Income									994.4	1,033.5	1,077.1
Net Investment Income									1,370.3	1,404.9	1,377.1
Other Income									66.6	71.3	75.4
Adjusted Operating Revenue									2,431.3	2,509.7	2,529.6
Adjusted Operating Income (Loss)									241.4	137.7	125.5
Interest and Debt Expense									3.1	5.3	6.9
Depreciation and Amortization									5.9	7.7	8.3
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Premium Income									0.0	0.0	0.0
Net Investment Income									9.8	20.5	29.5
Other Income									1.1	3.1	2.7
Adjusted Operating Revenue									10.9	23.6	32.2
Adjusted Operating Income (Loss)									(200.8)	(188.6)	(171.1)
Interest and Debt Expense									185.1	172.1	160.4
Depreciation and Amortization									$ 0.7	$ 1.7	$ 1.0