Treasury Stock Transactions (Details) - USD ($) shares in Millions, $ in Millions		1 Months Ended				3 Months Ended		6 Months Ended	7 Months Ended	8 Months Ended	9 Months Ended
		Feb. 28, 2025	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Jul. 31, 2024	Mar. 31, 2025	Sep. 30, 2025		Sep. 30, 2024	Nov. 01, 2024	Jul. 02, 2024	Apr. 02, 2024	Jan. 03, 2024
Equity, Class of Treasury Stock
Cost of Shares Repurchased	[1]					$ 253.3	$ 202.0				$ 759.2		$ 504.8
Number of Treasury Shares Repurchased						3.2	3.7				10.3	[2]	9.7
Commissions Paid on Stock Repurchases						$ 0.9					$ 1.8
Share Repurchases Excise Tax						2.4	$ 2.0				$ 7.4		$ 4.8
Accelerated Share Repurchases, Cash or Stock Settlement											Under the terms of the agreements, we are to receive, or be required to pay, a price adjustment based on the volume weighted average price of Unum Group common stock during the term of the agreement, less a discount. Any price adjustment payable to us is settled in shares of Unum Group common stock. Any price adjustment we would be required to pay may be settled in either cash or common stock at our option
November 2024 Accelerated Share Repurchase
Equity, Class of Treasury Stock
Number of Treasury Shares Repurchased	[3]	0.7
ASR prepayment amount	[3]													$ 321.0
July 2024 Accelerated Share Repurchase
Equity, Class of Treasury Stock
Number of Treasury Shares Repurchased			0.6
ASR prepayment amount															$ 150.0
April 2024 Accelerated Share Repurchase
Equity, Class of Treasury Stock
Number of Treasury Shares Repurchased				0.7
ASR prepayment amount																$ 125.0
January 2024 Accelerated Share Repurchase
Equity, Class of Treasury Stock
Number of Treasury Shares Repurchased					0.5
ASR prepayment amount																	$ 100.0
Repurchase Program Authorized in October 2023
Equity, Class of Treasury Stock
Share Repurchase Program, Expiration Date	[4]										Jul. 31, 2024
Authorized Amount of Treasury Share Repurchases Under Stock Repurchase Program	[4]					500.0		$ 500.0			$ 500.0
Cost of Shares Repurchased									$ 464.2
Remaining Amount of Treasury Share Repurchases Authorized Under Stock Repurchase Program	[4]					0.0		0.0			$ 0.0
Repurchase Program Authorized in July 2024
Equity, Class of Treasury Stock
Share Repurchase Program, Expiration Date	[5]										Mar. 31, 2025
Authorized Amount of Treasury Share Repurchases Under Stock Repurchase Program	[5]					1,000.0		1,000.0			$ 1,000.0
Cost of Shares Repurchased										$ 706.8
Remaining Amount of Treasury Share Repurchases Authorized Under Stock Repurchase Program	[5]					0.0		0.0			0.0
Repurchase Program Authorized in February 2025
Equity, Class of Treasury Stock
Authorized Amount of Treasury Share Repurchases Under Stock Repurchase Program						1,000.0		1,000.0			1,000.0
Cost of Shares Repurchased								550.0
Remaining Amount of Treasury Share Repurchases Authorized Under Stock Repurchase Program						$ 450.0		$ 450.0			$ 450.0

[1] 2Includes $0.9 million and $1.8 million of commissions for the three and nine months ended September 30, 2025, respectively, and a de minimis amount of commissions for the three and nine months ended September 30, 2024. Also includes $2.4 million and $7.4 million of excise tax for the three and nine months ended September 30, 2025, respectively, and $2.0 million and $4.8 million of excise tax for the three and nine months ended September 30, 2024, respectively.

[2] 1For the nine months ended September 30, 2025, includes 0.7 million shares related to the settlement of the November 2024 accelerated share repurchase agreement (ASR) which occurred in February 2025.

[3] The final price adjustment settlement, along with the delivery of the remaining shares, occurred in February 2025, resulting in the delivery to us of 0.7 million additional shares. As a result of the final settlement occurring subsequent to December 31, 2024, we recorded a decrease of $80.3 million to additional paid-in capital within stockholders' equity on our consolidated balance sheet for the value of the shares held back by the counterparty as of December 31, 2024, which was reclassified to treasury stock in the first quarter of 2025 in connection with the final settlement of the agreement.

[4] 2Concurrent with the announcement of the July 2024 repurchase program, we also announced the termination of the October 2023 program as of July 31, 2024, and all unused amounts under that program expired as of that date.

[5] 1Concurrent with the announcement of the February 2025 repurchase program, we also announced the termination of the July 2024 program as of March 31, 2025, and all unused amounts under that program expired as of that date.