Policyholder Account Balance (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2026		Sep. 30, 2025		Dec. 31, 2025
Policyholder Account Balance [Roll Forward]
Policyholders' Account Balance	$ 6,494.8				$ 5,636.4
Unum US
Policyholder Account Balance [Roll Forward]
Policyholders' Account Balance	651.9
Unum US | Voluntary Benefits
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	548.4		$ 568.8
Policyholder Account Balance, Premium Received	33.3		37.3
Policyholder Account Balance, Policy Charge	(38.7)	[1]	(41.0)	[2]
Policyholder Account Balance, Surrender and Withdrawal	(26.6)		(30.0)
Policyholder Account Balance, Benefit Payment	(4.5)		(4.6)
Policyholder Account Balance, Interest Expense	14.0		15.1
Policyholder Account Balance, Increase (Decrease) from Other Change	5.7		7.2
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	531.6		552.8
Policyholder Account Balance, Reserves in excess of Account Balance	120.3		114.6
Policyholders' Account Balance	651.9		667.4
Policyholder Account Balance, Reinsurance recoverable	0.6		0.7
Policyholder Account Balance, After reinsurance recoverable	$ 651.3		$ 666.7
Policyholder Account Balance, Weighted Average Crediting Rate	3.50%		3.60%
Policyholder Account Balance, Net Amount at Risk	$ 3,480.7	[3]	$ 3,822.1	[4]
Policyholder Account Balance, Cash Surrender Value	523.1		542.5
Colonial Life
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	845.2		849.0
Policyholder Account Balance, Premium Received	53.4		56.3
Policyholder Account Balance, Policy Charge	(50.6)	[1]	(52.2)	[2]
Policyholder Account Balance, Surrender and Withdrawal	(26.6)		(28.3)
Policyholder Account Balance, Benefit Payment	(6.1)		(5.9)
Policyholder Account Balance, Interest Expense	25.3		25.4
Policyholder Account Balance, Increase (Decrease) from Other Change	(0.3)		0.1
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	840.3		844.4
Policyholder Account Balance, Reserves in excess of Account Balance	12.7		12.9
Policyholders' Account Balance	853.0		857.3
Policyholder Account Balance, Reinsurance recoverable	0.0		0.0
Policyholder Account Balance, After reinsurance recoverable	$ 853.0		$ 857.3
Policyholder Account Balance, Weighted Average Crediting Rate	4.10%		4.10%
Policyholder Account Balance, Net Amount at Risk	$ 7,306.6	[3]	$ 7,795.1	[4]
Policyholder Account Balance, Cash Surrender Value	824.7		821.9
Closed Block
Policyholder Account Balance [Roll Forward]
Policyholders' Account Balance	3,995.2
Closed Block | Other Insurance Product Line
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	4,073.4		4,052.2
Policyholder Account Balance, Premium Received	20.1		24.2
Policyholder Account Balance, Policy Charge	(79.0)	[1]	(82.5)	[2]
Policyholder Account Balance, Surrender and Withdrawal	(165.2)		(12.4)
Policyholder Account Balance, Benefit Payment	(130.9)		(136.6)
Policyholder Account Balance, Interest Expense	233.3		247.4
Policyholder Account Balance, Increase (Decrease) from Other Change	3.2		0.7
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	3,954.9		4,093.0
Policyholder Account Balance, Reserves in excess of Account Balance	40.3		42.2
Policyholders' Account Balance	3,995.2		4,135.2
Policyholder Account Balance, Reinsurance recoverable	3,995.2		4,135.2
Policyholder Account Balance, After reinsurance recoverable	$ 0.0		$ 0.0
Policyholder Account Balance, Weighted Average Crediting Rate	8.00%		8.50%
Policyholder Account Balance, Net Amount at Risk	$ 1,473.3	[3]	$ 1,610.8	[4]
Policyholder Account Balance, Cash Surrender Value	3,916.0		4,057.3
Funding Agreements
Policyholder Account Balance [Roll Forward]
Policyholders' Account Balance	994.7
Consolidated
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	5,467.0		5,470.0
Policyholder Account Balance, Premium Received	106.8		117.8
Policyholder Account Balance, Policy Charge	(168.3)	[1]	(175.7)	[2]
Policyholder Account Balance, Surrender and Withdrawal	(218.4)		(70.7)
Policyholder Account Balance, Benefit Payment	(141.5)		(147.1)
Policyholder Account Balance, Interest Expense	272.6		287.9
Policyholder Account Balance, Increase (Decrease) from Other Change	8.6		8.0
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	5,326.8		5,490.2
Policyholder Account Balance, Reserves in excess of Account Balance	173.3		169.7
Policyholders' Account Balance	5,500.1		5,659.9
Policyholder Account Balance, Reinsurance recoverable	3,995.8		4,135.9
Policyholder Account Balance, After reinsurance recoverable	$ 1,504.3		$ 1,524.0
Policyholder Account Balance, Weighted Average Crediting Rate	6.90%		7.30%
Policyholder Account Balance, Net Amount at Risk	$ 12,260.6	[3]	$ 13,228.0	[4]
Policyholder Account Balance, Cash Surrender Value	$ 5,263.8		$ 5,421.7

[1] 1Contracts included in the policyholders' account balances are generally charged a premium and/or monthly assessments on the basis of the account balance.

[2] 1Contracts included in the policyholders' account balances are generally charged a premium and/or monthly assessments on the basis of the account balance.

[3] 2For those guarantees of benefits that are payable in the event of death, the net amount at risk is generally defined as the current guaranteed minimum death benefit in excess of the current account balance at the balance sheet date.

[4] 2For those guarantees of benefits that are payable in the event of death, the net amount at risk is generally defined as the current guaranteed minimum death benefit in excess of the current account balance at the balance sheet date.