Schedule III Supplementary Insurance Information (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		$ 2,842.8	$ 2,714.5
Reserves for Future Policy Contract Benefits		36,806.4	40,009.4
Policyholder Account Balances		5,633.7	5,667.7
Unearned Premiums		384.0	380.2
Premium Income		10,497.4	10,046.0	$ 9,616.5
Net Investment Income	[1]	2,130.0	2,096.7	2,122.2
Benefits and Change in Reserves for Future Benefits	[2]	6,917.9	7,257.1	6,994.6
Amortization of Deferred Acquisition Costs		521.0	481.4	421.1
All Other Expenses	[3]	3,197.1	3,007.3	2,818.4
Unum US
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,260.6	1,232.2
Reserves for Future Policy Contract Benefits		8,669.7	9,419.1
Policyholder Account Balances		675.7	678.1
Unearned Premiums		51.7	54.6
Premium Income		6,883.2	6,579.2	6,251.4
Net Investment Income	[1]	632.2	639.9	676.3
Benefits and Change in Reserves for Future Benefits	[2]	3,864.7	3,808.5	3,970.9
Amortization of Deferred Acquisition Costs		292.5	267.6	240.9
All Other Expenses	[3]	2,011.3	1,879.2	1,768.8
Premiums Written	[4]	4,834.0	4,634.3	4,335.7
Unum International
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		53.0	46.9
Reserves for Future Policy Contract Benefits		2,163.0	2,305.3
Policyholder Account Balances		0.0	0.0
Unearned Premiums		165.5	151.4
Premium Income		949.5	825.2	718.8
Net Investment Income	[1]	128.8	137.2	170.1
Benefits and Change in Reserves for Future Benefits	[2]	656.7	579.8	549.6
Amortization of Deferred Acquisition Costs		9.5	8.4	8.2
All Other Expenses	[3]	263.4	235.6	190.4
Premiums Written	[4]	581.6	544.4	504.1
Colonial Life
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,529.2	1,435.4
Reserves for Future Policy Contract Benefits		1,904.2	1,997.8
Policyholder Account Balances		862.5	869.8
Unearned Premiums		45.4	44.5
Premium Income		1,783.9	1,726.1	1,702.0
Net Investment Income	[1]	161.5	153.5	152.7
Benefits and Change in Reserves for Future Benefits	[2]	804.7	798.1	826.1
Amortization of Deferred Acquisition Costs		219.0	205.4	172.0
All Other Expenses	[3]	413.0	396.5	389.6
Premiums Written	[4]	1,247.4	1,221.8	1,221.8
Closed Block
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		0.0	0.0
Reserves for Future Policy Contract Benefits		24,069.5	26,287.2
Policyholder Account Balances		4,095.5	4,119.8
Unearned Premiums		121.4	129.7
Premium Income		880.8	915.5	944.3
Net Investment Income	[1]	1,148.9	1,066.3	1,070.6
Benefits and Change in Reserves for Future Benefits	[2]	1,591.8	2,070.7	1,648.0
Amortization of Deferred Acquisition Costs		0.0	0.0	0.0
All Other Expenses	[3]	243.0	246.5	248.0
Premiums Written	[4]	868.1	904.2	936.8
Corporate and Other Segment
Supplementary Insurance Information, by Segment
Premium Income		0.0	0.0	0.0
Net Investment Income	[1]	58.6	99.8	52.5
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	[3]	266.4	249.5	221.6
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		41.4	44.1	50.3
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Supplementary Insurance Information, by Segment
Non-Contemporaneous Reinsurance		$ 25.1	$ 34.8	$ 34.4

[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] Included in policy benefits including remeasurement loss or gain were the following: •In 2024, reserve assumption updates resulting in a net decrease in policy benefits including remeasurement gain or loss in the Unum US, Colonial Life and Closed Block segments of $143.6 million, $46.0 million, $175.3 million, respectively and a net increase in policy benefits including remeasurement gain or loss in the Unum International segment of $7.5 million. •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, and $55.2 million, and $9.7 million respectively. •In 2024, 2023, and 2022 the impact of non-contemporaneous reinsurance of $25.1 million, $34.8 million, and $34.4 million respectively, in the Closed Block segment.

[3] 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 2024, 2023 and 2022 the amortization of the cost of reinsurance of $41.4 million, $44.1 million, and $50.3 million respectively, in the Closed Block segment. •In 2024, we incurred a loss on legal settlement of $15.3 million for the settlement of an employment-related matter, in the Corporate segment.

[4] Excludes life insurance.