Schedule III Supplementary Insurance Information (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		$ 2,920.3	$ 2,842.8
Reserves for Future Policy Contract Benefits		38,017.0	36,806.4
Policyholder Account Balances		5,636.4	5,633.7
Unearned Premiums		412.8	384.0
Premium Income		10,831.0	10,497.4	$ 10,046.0
Net Investment Income	[1]	2,032.7	2,130.0	2,096.7
Benefits and Change in Reserves for Future Benefits	[2]	8,065.3	6,917.9	7,257.1
Amortization of Deferred Acquisition Costs		527.1	521.0	481.4
All Other Expenses	[3]	3,549.6	3,197.1	3,007.3
Amortization of Deferred Loss on Reinsurance		116.7	41.4	44.1
Accelerated Charitable Contribution		20.0	0.0	0.0
Settlement Loss on the U.S. Pension Plan Annuity Purchase		103.8	0.0	0.0
Loss on Legal Settlement			15.3
Pension Plan | UNITED STATES
Supplementary Insurance Information, by Segment
Settlement Loss on the U.S. Pension Plan Annuity Purchase		103.8	0.0	0.0
Unum US
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,208.7	1,260.6
Reserves for Future Policy Contract Benefits		8,493.4	8,669.7
Policyholder Account Balances		665.7	675.7
Unearned Premiums		48.7	51.7
Premium Income		7,083.5	6,883.2	6,579.2
Net Investment Income	[1]	604.2	632.2	639.9
Benefits and Change in Reserves for Future Benefits	[2]	4,120.4	3,864.7	3,808.5
Amortization of Deferred Acquisition Costs		283.3	292.5	267.6
All Other Expenses	[3]	2,115.2	2,011.3	1,879.2
Premiums Written	[4]	4,947.5	4,834.0	4,634.3
Reserve Assumption Updates		147.7	143.6	128.8
Unum International
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		72.1	53.0
Reserves for Future Policy Contract Benefits		2,404.3	2,163.0
Policyholder Account Balances		0.0	0.0
Unearned Premiums		201.5	165.5
Premium Income		1,082.8	949.5	825.2
Net Investment Income	[1]	145.6	128.8	137.2
Benefits and Change in Reserves for Future Benefits	[2]	767.4	656.7	579.8
Amortization of Deferred Acquisition Costs		10.7	9.5	8.4
All Other Expenses	[3]	300.3	263.4	235.6
Premiums Written	[4]	613.6	581.6	544.4
Reserve Assumption Updates		5.4	(7.5)	(17.9)
Colonial Life
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,639.5	1,529.2
Reserves for Future Policy Contract Benefits		2,032.0	1,904.2
Policyholder Account Balances		858.9	862.5
Unearned Premiums		46.9	45.4
Premium Income		1,839.1	1,783.9	1,726.1
Net Investment Income	[1]	172.6	161.5	153.5
Benefits and Change in Reserves for Future Benefits	[2]	876.2	804.7	798.1
Amortization of Deferred Acquisition Costs		233.1	219.0	205.4
All Other Expenses	[3]	431.5	413.0	396.5
Premiums Written	[4]	1,278.1	1,247.4	1,221.8
Reserve Assumption Updates		8.9	46.0	80.7
Closed Block
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		0.0	0.0
Reserves for Future Policy Contract Benefits		25,087.3	24,069.5
Policyholder Account Balances		4,111.8	4,095.5
Unearned Premiums		115.7	121.4
Premium Income		825.6	880.8	915.5
Net Investment Income	[1]	1,016.5	1,148.9	1,066.3
Benefits and Change in Reserves for Future Benefits	[2]	2,301.3	1,591.8	2,070.7
Amortization of Deferred Acquisition Costs		0.0	0.0	0.0
All Other Expenses	[3]	313.0	243.0	246.5
Premiums Written	[4]	818.8	868.1	904.2
Reserve Assumption Updates		(640.5)	175.3	(368.8)
Corporate and Other Segment
Supplementary Insurance Information, by Segment
Premium Income		0.0	0.0	0.0
Net Investment Income	[1]	93.8	58.6	99.8
Benefits and Change in Reserves for Future Benefits	[2]	0.0	0.0	0.0
Amortization of Deferred Acquisition Costs		0.0	0.0	0.0
All Other Expenses	[3]	389.6	266.4	249.5
Premiums Written	[4]	0.0	0.0	0.0
Operating Segments
Supplementary Insurance Information, by Segment
Reserve Assumption Updates		(478.5)	357.4	(177.2)
Loss on Legal Settlement		0.0	15.3	0.0
Operating Segments | Unum US
Supplementary Insurance Information, by Segment
Non-Contemporaneous Reinsurance		(1.0)
Operating Segments | Closed Block
Supplementary Insurance Information, by Segment
Non-Contemporaneous Reinsurance		$ (28.6)	$ 25.1	$ 34.8

[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 2025, reserve assumption updates resulting in a net decrease in policy benefits including remeasurement gain or loss in the Unum US, Colonial Life and Unum International segments of $147.7 million, $8.9 million, $5.4 million, respectively, and a net increase in policy benefits including remeasurement gain or loss in the Closed Block segment of $640.5 million. •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, and $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 2025, 2024, and 2023 the impact of non-contemporaneous reinsurance of $28.6 million, $25.1 million, and $34.8 million, respectively, in the Closed Block segment. In 2025, the impact of non-contemporaneous reinsurance of $1.0 million in the Unum US 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 2025, 2024 and 2023 the amortization of the cost of reinsurance of $116.7 million, $41.4 million, and $44.1 million respectively, in the Closed Block segment. •In 2025, we incurred an expense related to an accelerated charitable contribution of $20.0 million, in the Corporate segment. •In 2025, we incurred a settlement loss on the U.S. pension plan annuity purchase of $103.8 million, in the Corporate 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