Policyholder Account Balance (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024		Mar. 31, 2023		Dec. 31, 2023
Policyholder Account Balance [Roll Forward]
Policyholders' Account Balances	$ 5,639.0				$ 5,667.7
Operating Segments
Policyholder Account Balance [Roll Forward]
Balance, beginning of year	5,514.2		$ 5,598.6
Premiums received	40.2		44.0
Policy Charges	(60.6)	[1]	(60.8)	[2]
Surrenders and withdrawals	(22.8)		(24.3)
Benefit payments	(79.5)		(70.5)
Interest credited	95.6		94.0
Other	3.2		(15.4)
Balance, end of period	5,490.3		5,565.6
Reserves in excess of account balance	148.7		144.0
Policyholders' Account Balances	5,639.0		5,709.6
Policyholder Account Balance, Reinsurance recoverable	4,097.9		4,166.1
Net policyholders' account balances, after reinsurance recoverable	$ 1,541.1		$ 1,543.5
Weighted average crediting rate	7.20%		6.90%
Net Amount at Risk	$ 14,828.5	[3]	$ 15,820.0	[4]
Cash surrender value	5,425.2		5,413.5
Colonial Life
Policyholder Account Balance [Roll Forward]
Balance, beginning of year	852.9		852.4
Premiums received	20.5		22.3
Policy Charges	(18.3)	[1]	(19.0)	[2]
Surrenders and withdrawals	(9.7)		(10.2)
Benefit payments	(2.1)		(2.2)
Interest credited	8.6		8.6
Other	0.1		(0.2)
Balance, end of period	852.0		851.7
Reserves in excess of account balance	16.2		15.8
Policyholders' Account Balances	868.2		867.5
Policyholder Account Balance, Reinsurance recoverable	0.0		0.2
Net policyholders' account balances, after reinsurance recoverable	$ 868.2		$ 867.3
Weighted average crediting rate	4.10%		4.00%
Net Amount at Risk	$ 8,621.0	[3]	$ 9,182.6	[4]
Cash surrender value	816.6		805.0
Voluntary Benefits | Unum US
Policyholder Account Balance [Roll Forward]
Balance, beginning of year	578.6		586.8
Premiums received	14.0		15.4
Policy Charges	(14.5)	[1]	(15.5)	[2]
Surrenders and withdrawals	(10.2)		(9.0)
Benefit payments	(1.9)		(3.0)
Interest credited	5.3		5.7
Other	2.8		2.0
Balance, end of period	574.1		582.4
Reserves in excess of account balance	99.7		94.8
Policyholders' Account Balances	673.8		677.2
Policyholder Account Balance, Reinsurance recoverable	0.9		1.0
Net policyholders' account balances, after reinsurance recoverable	$ 672.9		$ 676.2
Weighted average crediting rate	3.70%		3.90%
Net Amount at Risk	$ 4,393.5	[3]	$ 4,794.7	[4]
Cash surrender value	563.3		572.3
Other Insurance Product Line | Closed Block
Policyholder Account Balance [Roll Forward]
Balance, beginning of year	4,082.7		4,159.4
Premiums received	5.7		6.3
Policy Charges	(27.8)	[1]	(26.3)	[2]
Surrenders and withdrawals	(2.9)		(5.1)
Benefit payments	(75.5)		(65.3)
Interest credited	81.7		79.7
Other	0.3		(17.2)
Balance, end of period	4,064.2		4,131.5
Reserves in excess of account balance	32.8		33.4
Policyholders' Account Balances	4,097.0		4,164.9
Policyholder Account Balance, Reinsurance recoverable	4,097.0		4,164.9
Net policyholders' account balances, after reinsurance recoverable	$ 0.0		$ 0.0
Weighted average crediting rate	8.30%		7.90%
Net Amount at Risk	$ 1,814.0	[3]	$ 1,842.7	[4]
Cash surrender value	$ 4,045.3		$ 4,036.2

[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.