Policyholder Account Balance (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2023		Jun. 30, 2022		Dec. 31, 2022	[2]
Policyholder Account Balance [Roll Forward]
Policyholders' Account Balances	$ 5,692.4		$ 5,758.7	[1]	$ 5,740.2
Operating Segments
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	5,598.6		5,679.6
Policyholder Account Balance, Premium Received	85.1		90.3
Policyholder Account Balance, Policy Charge	(121.3)	[3]	(124.8)	[4]
Policyholder Account Balance, Surrender and Withdrawal	(44.5)		(51.7)
Policyholder Account Balance, Benefit Payment	(143.3)		(146.2)
Policyholder Account Balance, Interest Expense	187.3		182.6
Policyholder Account Balance, Increase (Decrease) from Other Change	(14.5)		(9.1)
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	5,547.4		5,620.7
Policyholder Account Balance, Reserves in excess of Account Balance	145.0		138.0
Policyholders' Account Balances	5,692.4		5,758.7
Policyholder Account Balance, Reinsurance recoverable	4,149.2		4,214.4
Policyholder Account Balance, After reinsurance recoverable	$ 1,543.2		$ 1,544.3
Policyholder Account Balance, Weighted Average Crediting Rate	6.90%		6.60%
Policyholder Account Balance, Net Amount at Risk	$ 15,581.8	[5]	$ 16,615.1	[6]
Policyholder Account Balance, Cash Surrender Value	5,399.5		5,459.6
Unum US | Voluntary Benefits
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	586.8		598.7
Policyholder Account Balance, Premium Received	30.2		33.4
Policyholder Account Balance, Policy Charge	(30.7)	[3]	(33.0)	[4]
Policyholder Account Balance, Surrender and Withdrawal	(16.4)		(17.9)
Policyholder Account Balance, Benefit Payment	(5.4)		(6.0)
Policyholder Account Balance, Interest Expense	11.4		12.0
Policyholder Account Balance, Increase (Decrease) from Other Change	4.6		3.7
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	580.5		590.9
Policyholder Account Balance, Reserves in excess of Account Balance	96.7		86.4
Policyholders' Account Balances	677.2		677.3
Policyholder Account Balance, Reinsurance recoverable	1.0		1.1
Policyholder Account Balance, After reinsurance recoverable	$ 676.2		$ 676.2
Policyholder Account Balance, Weighted Average Crediting Rate	4.00%		4.10%
Policyholder Account Balance, Net Amount at Risk	$ 4,695.5	[5]	$ 5,117.2	[6]
Policyholder Account Balance, Cash Surrender Value	570.3		586.9
Colonial Life
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	852.4		849.2
Policyholder Account Balance, Premium Received	43.8		46.5
Policyholder Account Balance, Policy Charge	(37.8)	[3]	(39.5)	[4]
Policyholder Account Balance, Surrender and Withdrawal	(19.2)		(19.0)
Policyholder Account Balance, Benefit Payment	(4.1)		(4.3)
Policyholder Account Balance, Interest Expense	17.1		17.1
Policyholder Account Balance, Increase (Decrease) from Other Change	(0.1)		0.0
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	852.1		850.0
Policyholder Account Balance, Reserves in excess of Account Balance	14.9		18.3
Policyholders' Account Balances	867.0		868.3
Policyholder Account Balance, Reinsurance recoverable	0.0		0.2
Policyholder Account Balance, After reinsurance recoverable	$ 867.0		$ 868.1
Policyholder Account Balance, Weighted Average Crediting Rate	4.10%		4.10%
Policyholder Account Balance, Net Amount at Risk	$ 9,042.3	[5]	$ 9,648.2	[6]
Policyholder Account Balance, Cash Surrender Value	807.7		793.0
Closed Block | Other Insurance Product Line
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	4,159.4		4,231.7
Policyholder Account Balance, Premium Received	11.1		10.4
Policyholder Account Balance, Policy Charge	(52.8)	[3]	(52.3)	[4]
Policyholder Account Balance, Surrender and Withdrawal	(8.9)		(14.8)
Policyholder Account Balance, Benefit Payment	(133.8)		(135.9)
Policyholder Account Balance, Interest Expense	158.8		153.5
Policyholder Account Balance, Increase (Decrease) from Other Change	(19.0)		(12.8)
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	4,114.8		4,179.8
Policyholder Account Balance, Reserves in excess of Account Balance	33.4		33.3
Policyholders' Account Balances	4,148.2		4,213.1
Policyholder Account Balance, Reinsurance recoverable	4,148.2		4,213.1
Policyholder Account Balance, After reinsurance recoverable	$ 0.0		$ 0.0
Policyholder Account Balance, Weighted Average Crediting Rate	7.90%		7.50%
Policyholder Account Balance, Net Amount at Risk	$ 1,844.0	[5]	$ 1,849.7	[6]
Policyholder Account Balance, Cash Surrender Value	$ 4,021.5		$ 4,079.7

[1] 2We previously reported policy and contract benefits and reserves for future policy and contract benefits liabilities in our consolidated balance sheet as of June 30, 2022 of $1,815.9 million and $43,088.0 million, respectively, resulting in total policyholder liabilities of $44,903.9 million. These balances were reclassified into new line items, future policy benefits and policyholders' account balances, which would have reported balances of $39,154.8 million and $5,749.1 million, respectively, as of June 30, 2022, resulting in total policyholder liabilities of $44,903.9 million under the historical accounting method.

[2] 1We previously reported policy and contract benefits and reserves for future policy and contract benefits liabilities in our consolidated balance sheet as of December 31, 2022 of $1,839.8 million and 42,330.2 million, respectively, resulting in total policyholder liabilities of $44,170.0 million. These balances were reclassified into new line items, future policy benefits and policyholders' account balances, which would have reported balances of $38,443.2 million and $5,726.8 million, respectively, as of December 31, 2022, resulting in total policyholder liabilities of $44,170.0 million under the historical accounting method.

[3] 1Contracts included in the policyholder account balances are generally charged a premium and/or monthly assessments on the basis of the account balance.

[4] 1Contracts included in the policyholder account balances are generally charged a premium and/or monthly assessments on the basis of the account balance.

[5] 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.

[6] 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.