Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information
Premium Income	$ 10,046.0	$ 9,616.5	$ 9,475.0
Net Investment Income	2,096.7	2,122.2	2,213.2
Other Income	279.2	261.1	242.9
Interest and Debt Expense	194.8	188.5	185.0
Cost Related to Early Retirement of Debt	0.0	4.2	67.3
Amortization of Deferred Acquisition Costs	481.4	421.1	452.1
Unum US
Segment Reporting Information
Amortization of Deferred Acquisition Costs	267.6	240.9	285.9
Unum International
Segment Reporting Information
Amortization of Deferred Acquisition Costs	8.4	8.2	7.0
Colonial Life
Segment Reporting Information
Amortization of Deferred Acquisition Costs	205.4	172.0	159.2
Operating Segments
Segment Reporting Information
Premium Income	10,046.0	9,616.5	9,475.0
Net Investment Income	2,096.7	2,122.2	2,213.2
Other Income	279.2	261.1	242.9
Adjusted Operating Revenue	12,421.9	11,999.8	11,931.1
Adjusted Operating Income (Loss)	1,932.2	1,607.1	1,151.1
Interest and Debt Expense	194.8	188.5	185.0
Cost Related to Early Retirement of Debt		4.2	67.3
Amortization of Deferred Acquisition Costs	481.4	421.1	452.1
Depreciation and Amortization	110.3	110.4	117.0
Operating Segments | Unum US
Segment Reporting Information
Premium Income	6,579.2	6,251.4	6,072.0
Net Investment Income	639.9	676.3	721.6
Other Income	220.5	196.3	170.0
Adjusted Operating Revenue	7,439.6	7,124.0	6,963.6
Adjusted Operating Income (Loss)	1,355.5	972.6	398.9
Interest and Debt Expense	0.0	0.0	0.0
Cost Related to Early Retirement of Debt		0.0	0.0
Amortization of Deferred Acquisition Costs	267.6	240.9	285.9
Depreciation and Amortization	76.3	75.7	81.9
Operating Segments | Unum International
Segment Reporting Information
Premium Income	825.2	718.8	717.0
Net Investment Income	137.2	170.1	132.7
Other Income	1.6	0.9	0.6
Adjusted Operating Revenue	964.0	889.8	850.3
Adjusted Operating Income (Loss)	158.1	134.0	105.7
Interest and Debt Expense	0.0	0.0	0.0
Cost Related to Early Retirement of Debt		0.0	0.0
Amortization of Deferred Acquisition Costs	8.4	8.2	7.0
Depreciation and Amortization	14.6	14.4	13.0
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	1,726.1	1,702.0	1,690.2
Net Investment Income	153.5	152.7	172.0
Other Income	1.2	1.1	1.0
Adjusted Operating Revenue	1,880.8	1,855.8	1,863.2
Adjusted Operating Income (Loss)	400.1	412.9	404.3
Interest and Debt Expense	0.0	0.0	0.0
Cost Related to Early Retirement of Debt		0.0	0.0
Amortization of Deferred Acquisition Costs	205.4	172.0	159.2
Depreciation and Amortization	13.6	15.0	16.3
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	915.5	944.3	995.8
Net Investment Income	1,066.3	1,070.6	1,159.0
Other Income	52.6	58.0	65.1
Adjusted Operating Revenue	2,034.4	2,072.9	2,219.9
Adjusted Operating Income (Loss)	164.9	251.9	420.1
Interest and Debt Expense	0.0	0.0	0.0
Cost Related to Early Retirement of Debt		0.0	0.0
Amortization of Deferred Acquisition Costs	0.0	0.0	0.0
Depreciation and Amortization	5.2	4.7	5.1
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Premium Income	0.0	0.0	0.0
Net Investment Income	99.8	52.5	27.9
Other Income	3.3	4.8	6.2
Adjusted Operating Revenue	103.1	57.3	34.1
Adjusted Operating Income (Loss)	(146.4)	(164.3)	(177.9)
Interest and Debt Expense	194.8	188.5	185.0
Amortization of Deferred Acquisition Costs	0.0	0.0	0.0
Depreciation and Amortization	$ 0.6	$ 0.6	$ 0.7