Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information
Premium Income	$ 9,481.0	$ 9,378.1	$ 9,365.6
Net Investment Income	2,213.2	2,360.7	2,435.3
Other Income	242.9	224.2	221.2
Interest and Debt Expense	185.0	188.2	177.4
Operating Segments
Segment Reporting Information
Premium Income	9,481.0	9,378.1	9,365.6
Net Investment Income	2,213.2	2,360.7	2,435.3
Other Income	242.9	224.2	221.2
Adjusted Operating Revenue	11,937.1	11,963.0	12,022.1
Adjusted Operating Income (Loss)	1,116.6	1,278.0	1,432.6
Interest and Debt Expense	185.0	188.2	177.4
Depreciation and Amortization	703.1	722.3	727.5
Operating Segments | Unum US
Segment Reporting Information
Premium Income	6,078.0	6,018.9	6,016.6
Other Income	170.0	154.9	142.8
Adjusted Operating Revenue	6,969.6	6,894.1	6,898.8
Adjusted Operating Income (Loss)	464.9	825.4	1,031.1
Interest and Debt Expense	0.0	0.0	0.0
Depreciation and Amortization	400.9	421.7	422.8
Operating Segments | Unum International
Segment Reporting Information
Premium Income	717.0	652.8	630.5
Net Investment Income	132.7
Other Income	0.6	0.5	0.6
Adjusted Operating Revenue	850.3	757.9	753.6
Adjusted Operating Income (Loss)	105.7	76.6	107.9
Interest and Debt Expense	0.0	0.0	0.0
Depreciation and Amortization	21.0	20.1	18.7
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	1,690.2	1,712.0	1,685.0
Other Income	1.0	1.1	3.4
Adjusted Operating Revenue	1,863.2	1,868.8	1,836.4
Adjusted Operating Income (Loss)	329.2	335.4	344.5
Interest and Debt Expense	0.0	0.0	0.0
Depreciation and Amortization	275.4	273.9	276.6
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	995.8	994.4	1,033.5
Other Income	65.1	66.6	71.3
Adjusted Operating Revenue	2,219.9	2,431.3	2,509.7
Adjusted Operating Income (Loss)	394.7	241.4	137.7
Interest and Debt Expense	0.0	3.1	5.3
Depreciation and Amortization	5.1	5.9	7.7
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Premium Income	0.0	0.0	0.0
Other Income	6.2	1.1	3.1
Adjusted Operating Revenue	34.1	10.9	23.6
Adjusted Operating Income (Loss)	(177.9)	(200.8)	(188.6)
Interest and Debt Expense	185.0	185.1	172.1
Depreciation and Amortization	$ 0.7	$ 0.7	$ 1.7