Policyholder Account Balance (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024		Sep. 30, 2023		Dec. 31, 2023
Policyholder Account Balance [Roll Forward]
Policyholders' Account Balances	$ 5,661.3				$ 5,667.7
Operating Segments
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	5,514.2		$ 5,598.6
Policyholder Account Balance, Premium Received	124.2		134.5
Policyholder Account Balance, Policy Charge	(170.8)	[1]	(181.0)	[2]
Policyholder Account Balance, Surrender and Withdrawal	(64.5)		(64.2)
Policyholder Account Balance, Benefit Payment	(167.9)		(204.1)
Policyholder Account Balance, Interest Expense	261.4		281.7
Policyholder Account Balance, Increase (Decrease) from Other Change	8.6		30.8
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	5,505.2		5,596.3
Policyholder Account Balance, Reserves in excess of Account Balance	156.1		148.2
Policyholders' Account Balances	5,661.3		5,744.5
Policyholder Account Balance, Reinsurance recoverable	4,124.3		4,199.3
Policyholder Account Balance, After reinsurance recoverable	$ 1,537.0		$ 1,545.2
Policyholder Account Balance, Weighted Average Crediting Rate	6.50%		6.90%
Policyholder Account Balance, Net Amount at Risk	$ 14,234.1	[3]	$ 15,308.7	[4]
Policyholder Account Balance, Cash Surrender Value	5,440.2		5,412.4
Unum US | Voluntary Benefits
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	578.6		586.8
Policyholder Account Balance, Premium Received	40.7		44.9
Policyholder Account Balance, Policy Charge	(42.9)	[1]	(45.8)	[2]
Policyholder Account Balance, Surrender and Withdrawal	(24.3)		(23.6)
Policyholder Account Balance, Benefit Payment	(4.8)		(7.5)
Policyholder Account Balance, Interest Expense	15.7		17.0
Policyholder Account Balance, Increase (Decrease) from Other Change	7.6		6.7
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	570.6		578.5
Policyholder Account Balance, Reserves in excess of Account Balance	104.4		99.2
Policyholders' Account Balances	675.0		677.7
Policyholder Account Balance, Reinsurance recoverable	0.8		1.0
Policyholder Account Balance, After reinsurance recoverable	$ 674.2		$ 676.7
Policyholder Account Balance, Weighted Average Crediting Rate	3.70%		4.00%
Policyholder Account Balance, Net Amount at Risk	$ 4,210.4	[3]	$ 4,600.0	[4]
Policyholder Account Balance, Cash Surrender Value	560.4		568.9
Colonial Life
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	852.9		852.4
Policyholder Account Balance, Premium Received	60.2		64.4
Policyholder Account Balance, Policy Charge	(54.2)	[1]	(56.3)	[2]
Policyholder Account Balance, Surrender and Withdrawal	(30.4)		(28.5)
Policyholder Account Balance, Benefit Payment	(6.0)		(5.6)
Policyholder Account Balance, Interest Expense	25.6		25.7
Policyholder Account Balance, Increase (Decrease) from Other Change	0.2		(0.2)
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	848.3		851.9
Policyholder Account Balance, Reserves in excess of Account Balance	14.5		16.6
Policyholders' Account Balances	862.8		868.5
Policyholder Account Balance, Reinsurance recoverable	0.0		0.0
Policyholder Account Balance, After reinsurance recoverable	$ 862.8		$ 868.5
Policyholder Account Balance, Weighted Average Crediting Rate	4.10%		4.10%
Policyholder Account Balance, Net Amount at Risk	$ 8,325.0	[3]	$ 8,901.6	[4]
Policyholder Account Balance, Cash Surrender Value	817.1		810.1
Closed Block | Other Insurance Product Line
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	4,082.7		4,159.4
Policyholder Account Balance, Premium Received	23.3		25.2
Policyholder Account Balance, Policy Charge	(73.7)	[1]	(78.9)	[2]
Policyholder Account Balance, Surrender and Withdrawal	(9.8)		(12.1)
Policyholder Account Balance, Benefit Payment	(157.1)		(191.0)
Policyholder Account Balance, Interest Expense	220.1		239.0
Policyholder Account Balance, Increase (Decrease) from Other Change	0.8		24.3
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	4,086.3		4,165.9
Policyholder Account Balance, Reserves in excess of Account Balance	37.2		32.4
Policyholders' Account Balances	4,123.5		4,198.3
Policyholder Account Balance, Reinsurance recoverable	4,123.5		4,198.3
Policyholder Account Balance, After reinsurance recoverable	$ 0.0		$ 0.0
Policyholder Account Balance, Weighted Average Crediting Rate	7.40%		8.00%
Policyholder Account Balance, Net Amount at Risk	$ 1,698.7	[3]	$ 1,807.1	[4]
Policyholder Account Balance, Cash Surrender Value	$ 4,062.7		$ 4,033.4

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

[2] 1Contracts included in the policyholder 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.