Policyholder Account Balance (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023		Mar. 31, 2022		Dec. 31, 2022		Dec. 31, 2021
Policyholder Account Balance [Line Items]
Policyholders' Account Balances	$ 5,709.6		$ 5,772.3	[1]	$ 5,740.2	[2]
Operating Segments
Policyholder Account Balance [Line Items]
Policyholders' Account Balances	5,709.6		5,772.3		5,598.6		$ 5,679.6
Policyholder Account Balance, Reinsurance recoverable	4,166.1		4,223.7
Policyholder Account Balance, After reinsurance recoverable	$ 1,543.5		$ 1,548.6
Policyholder Account Balance, Weighted Average Crediting Rate	6.90%		6.50%
Policyholder Account Balance, Net Amount at Risk	$ 15,820.0	[3]	$ 16,968.3
Policyholder Account Balance, Cash Surrender Value	5,413.5		5,427.7
Policyholder Account Balance, Premium Received	44.0		45.7
Policyholder Account Balance, Policy Charge	(60.8)	[4]	(64.4)
Policyholder Account Balance, Surrender and Withdrawal	(24.3)		(23.3)
Policyholder Account Balance, Benefit Payment	(70.5)		(74.6)
Policyholder Account Balance, Interest Expense	94.0		90.2
Policyholder Account Balance, Increase (Decrease) from Other Change	(15.4)		(22.9)
Policyholder Account Balance, before Reserves in excess of Account Balance	5,565.6		5,630.3
Policyholder Account Balance, Reserves in excess of Account Balance	144.0		142.0
Unum US | Voluntary Benefits
Policyholder Account Balance [Line Items]
Policyholders' Account Balances	677.2		680.6		586.8		598.7
Policyholder Account Balance, Reinsurance recoverable	1.0		1.2
Policyholder Account Balance, After reinsurance recoverable	$ 676.2		$ 679.4
Policyholder Account Balance, Weighted Average Crediting Rate	3.90%		4.60%
Policyholder Account Balance, Net Amount at Risk	$ 4,794.7	[3]	$ 5,219.2
Policyholder Account Balance, Cash Surrender Value	572.3		588.7
Policyholder Account Balance, Premium Received	15.4		17.2
Policyholder Account Balance, Policy Charge	(15.5)	[4]	(16.9)
Policyholder Account Balance, Surrender and Withdrawal	(9.0)		(10.2)
Policyholder Account Balance, Benefit Payment	(3.0)		(3.5)
Policyholder Account Balance, Interest Expense	5.7		6.9
Policyholder Account Balance, Increase (Decrease) from Other Change	2.0		2.2
Policyholder Account Balance, before Reserves in excess of Account Balance	582.4		594.4
Policyholder Account Balance, Reserves in excess of Account Balance	94.8		86.2
Colonial Life
Policyholder Account Balance [Line Items]
Policyholders' Account Balances	867.5		869.5		852.4		849.2
Policyholder Account Balance, Reinsurance recoverable	0.2		0.3
Policyholder Account Balance, After reinsurance recoverable	$ 867.3		$ 869.2
Policyholder Account Balance, Weighted Average Crediting Rate	4.00%		4.00%
Policyholder Account Balance, Net Amount at Risk	$ 9,182.6	[3]	$ 9,811.4
Policyholder Account Balance, Cash Surrender Value	805.0		789.3
Policyholder Account Balance, Premium Received	22.3		23.4
Policyholder Account Balance, Policy Charge	(19.0)	[4]	(20.0)
Policyholder Account Balance, Surrender and Withdrawal	(10.2)		(10.0)
Policyholder Account Balance, Benefit Payment	(2.2)		(2.3)
Policyholder Account Balance, Interest Expense	8.6		8.5
Policyholder Account Balance, Increase (Decrease) from Other Change	(0.2)		0.1
Policyholder Account Balance, before Reserves in excess of Account Balance	851.7		848.9
Policyholder Account Balance, Reserves in excess of Account Balance	15.8		20.6
Closed Block | Other Insurance Product Line
Policyholder Account Balance [Line Items]
Policyholders' Account Balances	4,164.9		4,222.2		$ 4,159.4		$ 4,231.7
Policyholder Account Balance, Reinsurance recoverable	4,164.9		4,222.2
Policyholder Account Balance, After reinsurance recoverable	$ 0.0		$ 0.0
Policyholder Account Balance, Weighted Average Crediting Rate	7.90%		7.30%
Policyholder Account Balance, Net Amount at Risk	$ 1,842.7	[3]	$ 1,937.7
Policyholder Account Balance, Cash Surrender Value	4,036.2		4,049.7
Policyholder Account Balance, Premium Received	6.3		5.1
Policyholder Account Balance, Policy Charge	(26.3)	[4]	(27.5)
Policyholder Account Balance, Surrender and Withdrawal	(5.1)		(3.1)
Policyholder Account Balance, Benefit Payment	(65.3)		(68.8)
Policyholder Account Balance, Interest Expense	79.7		74.8
Policyholder Account Balance, Increase (Decrease) from Other Change	(17.2)		(25.2)
Policyholder Account Balance, before Reserves in excess of Account Balance	4,131.5		4,187.0
Policyholder Account Balance, Reserves in excess of Account Balance	$ 33.4		$ 35.2

[1] 2We previously reported policy and contract benefits and reserves for future policy and contract benefits liabilities in our consolidated balance sheet as of March 31, 2022 of $1,874.2 million and $45,258.0 million, respectively, resulting in total policyholder liabilities of $47,132.2 million. These balances were reclassified into new line items, future policy benefits and policyholders' account balances, which would have reported balances of $41,368.2 million and $5,764.0 million, respectively, as of March 31, 2022, resulting in total policyholder liabilities of $47,132.2 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] 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] 1Contracts included in the policyholder account balances are generally charged a premium and/or monthly assessments on the basis of the account balance.