Schedule III Supplementary Insurance Information (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		$ 2,714.5	$ 2,560.0
Reserves for Future Policy Contract Benefits		40,009.4	38,577.1
Policyholder Account Balances		5,667.7	5,740.2
Unearned Premiums		380.2	365.5
Premium Income		10,046.0	9,616.5	$ 9,475.0
Net Investment Income	[1]	2,096.7	2,122.2	2,213.2
Benefits and Change in Reserves for Future Benefits		7,257.1	6,994.6	7,553.4
Amortization of Deferred Acquisition Costs		481.4	421.1	452.1
All Other Expenses	[2]	3,007.3	2,818.4	2,741.7
Unum US
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,232.2	1,185.1
Reserves for Future Policy Contract Benefits		9,419.1	9,710.3
Policyholder Account Balances		678.1	679.7
Unearned Premiums		54.6	55.8
Premium Income		6,579.2	6,251.4	6,072.0
Net Investment Income	[1]	639.9	676.3	721.6
Benefits and Change in Reserves for Future Benefits	[3]	3,808.5	3,970.9	4,430.3
Amortization of Deferred Acquisition Costs		267.6	240.9	285.9
All Other Expenses	[2]	1,879.2	1,768.8	1,616.8
Premiums Written	[4]	4,634.3	4,335.7	4,168.9
Unum International
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		46.9	37.0
Reserves for Future Policy Contract Benefits		2,305.3	2,063.3
Policyholder Account Balances		0.0	0.0
Unearned Premiums		151.4	130.4
Premium Income		825.2	718.8	717.0
Net Investment Income	[1]	137.2	170.1	132.7
Benefits and Change in Reserves for Future Benefits	[3]	579.8	549.6	553.0
Amortization of Deferred Acquisition Costs		8.4	8.2	7.0
All Other Expenses	[2]	235.6	190.4	180.4
Premiums Written	[4]	544.4	504.1	520.6
Colonial Life
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,435.4	1,337.9
Reserves for Future Policy Contract Benefits		1,997.8	1,858.4
Policyholder Account Balances		869.8	869.1
Unearned Premiums		44.5	42.7
Premium Income		1,726.1	1,702.0	1,690.2
Net Investment Income	[1]	153.5	152.7	172.0
Benefits and Change in Reserves for Future Benefits	[3]	798.1	826.1	911.7
Amortization of Deferred Acquisition Costs		205.4	172.0	159.2
All Other Expenses	[2]	396.5	389.6	364.5
Premiums Written	[4]	1,221.8	1,221.8	1,223.3
Closed Block
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		0.0	0.0
Reserves for Future Policy Contract Benefits		26,287.2	24,945.1
Policyholder Account Balances		4,119.8	4,191.4
Unearned Premiums		129.7	136.6
Premium Income		915.5	944.3	995.8
Net Investment Income	[1]	1,066.3	1,070.6	1,159.0
Benefits and Change in Reserves for Future Benefits	[3]	2,070.7	1,648.0	1,658.4
Amortization of Deferred Acquisition Costs		0.0	0.0	0.0
All Other Expenses	[2]	246.5	248.0	274.7
Premiums Written	[4]	904.2	936.8	983.0
Corporate and Other Segment
Supplementary Insurance Information, by Segment
Premium Income		0.0	0.0	0.0
Net Investment Income	[1]	99.8	52.5	27.9
Benefits and Change in Reserves for Future Benefits		0.0	0.0	0.0
Amortization of Deferred Acquisition Costs		0.0	0.0	0.0
All Other Expenses	[2]	249.5	221.6	305.3
Premiums Written	[4]	0.0	0.0	0.0
Closed Block Individual Disability Reinsurance Transaction
Supplementary Insurance Information, by Segment
Amortization of Prepaid Reinsurance Premium		44.1	50.3	69.8
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Supplementary Insurance Information, by Segment
Non-Contemporaneous Reinsurance		$ 34.8	34.4	$ 32.9
Amortization of Prepaid Reinsurance Premium			$ 50.3

[1] Net investment income is allocated based upon segmentation. Each segment has its own specifically identified assets and receives the investment income generated by those assets.

[2] Includes commissions, interest and debt expense, deferral of acquisition costs, compensation expense, and other expenses. Where not directly attributable to a segment, expenses are generally allocated based on activity levels, time information, and usage statistics. Also included in all other expenses were the following: •In 2023, 2022, and 2021 the amortization of the cost of reinsurance of $44.1 million, $50.3 million, and $69.8 million respectively, in the Closed Block segment. •In 2021, an impairment loss of $12.1 million was recorded in our Corporate segment for previously capitalized internal-use software that we no longer plan to utilize. •In 2021, costs related to early retirement of debt of $67.3 million in the Corporate segment. •In 2021, transaction costs of $6.2 million related to the Closed Block individual disability reinsurance transaction. •In 2021, a right-of-use asset impairment of $13.9 million related to one of our operating leases for office space that we do not plan to continue using to support general operations in the Corporate segment.

[3] Included in policy benefits including remeasurement loss or gain were the following: •In 2023, reserve assumption updates resulting in a net decrease in policy benefits including remeasurement gain or loss in the Unum US and Colonial Life segments of $128.8 million and $80.7 million, respectively and a net increase in policy benefits including remeasurement gain or loss in the Unum International and Closed Block segments of $17.9 million and $368.8 million, respectively. •In 2022, reserve assumption updates resulting in a net decrease in policy benefits including remeasurement gain or loss in the Unum US, Unum International, Colonial Life, and Closed Block segments of $170.8 million, $7.6 million, $55.2 million, and $9.7 million, respectively. •In 2021, reserve assumption updates resulting in a net decrease in policy benefits including remeasurement gain or loss in the Unum US, Unum International, and Colonial Life segments of $231.7 million, $4.2 million, and $23.5 million, respectively and a net increase in policy benefits including remeasurement gain or loss in the Closed Block segment of $24.4 million. •In 2023, 2022, and 2021 the impact of non-contemporaneous reinsurance of $34.8 million, $34.4 million, and $32.9 million respectively, in the Closed Block segment.

[4] Excludes life insurance.