Policyholders' Account Balances (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023		Dec. 31, 2022		Dec. 31, 2021
Policyholder Account Balance [Roll Forward]
Policyholders' Account Balances	$ 5,667.7		$ 5,740.2	[1]
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		$ 5,678.5
Policyholder Account Balance, Premium Received	175.1		187.6		204.5
Policyholder Account Balance, Policy Charge	(242.3)	[2]	(248.0)	[2]	(252.9)
Policyholder Account Balance, Surrender and Withdrawal	(88.9)		(90.4)		(89.5)
Policyholder Account Balance, Benefit Payment	(233.1)		(311.5)		(254.0)
Policyholder Account Balance, Interest Expense	378.5		369.2		373.1
Policyholder Account Balance, Increase (Decrease) from Other Change	(73.7)		12.1		19.9
Policyholder Account Balance, before Reserves in excess of Account Balance, end of period	5,514.2		5,598.6		5,679.6
Policyholder Account Balance, Reserves in excess of Account Balance	153.5		141.6		135.2
Policyholders' Account Balances	5,667.7		5,740.2		5,814.8
Policyholder Account Balance, Reinsurance recoverable	4,120.7		4,192.7		4,264.0
Policyholder Account Balance, After reinsurance recoverable	$ 1,547.0		$ 1,547.5		$ 1,550.8
Policyholder Account Balance, Weighted Average Crediting Rate	7.00%		6.70%		6.80%
Policyholder Account Balance, Net Amount at Risk	$ 15,080.0	[3]	$ 16,178.1	[3]	$ 17,408.7
Policyholder Account Balance, Cash Surrender Value	5,442.7		5,430.1		5,504.1
Colonial Life
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, beginning of year	852.4		849.2		843.8
Policyholder Account Balance, Premium Received	84.9		90.9		97.5
Policyholder Account Balance, Policy Charge	(74.7)	[2]	(78.0)	[2]	(82.4)
Policyholder Account Balance, Surrender and Withdrawal	(36.8)		(36.2)		(35.7)
Policyholder Account Balance, Benefit Payment	(7.2)		(8.0)		(8.5)
Policyholder Account Balance, Interest Expense	34.2		34.2		34.0
Policyholder Account Balance, Increase (Decrease) from Other Change	0.1		0.3		0.5
Policyholder Account Balance, before Reserves in excess of Account Balance, end of period	852.9		852.4		849.2
Policyholder Account Balance, Reserves in excess of Account Balance	16.9		16.7		21.5
Policyholders' Account Balances	869.8		869.1		870.7
Policyholder Account Balance, Reinsurance recoverable	0.0		0.2		0.3
Policyholder Account Balance, After reinsurance recoverable	$ 869.8		$ 868.9		$ 870.4
Policyholder Account Balance, Weighted Average Crediting Rate	4.10%		4.10%		4.10%
Policyholder Account Balance, Net Amount at Risk	$ 8,760.1	[3]	$ 9,338.5	[3]	$ 10,000.6
Policyholder Account Balance, Cash Surrender Value	813.5		800.9		784.6
Voluntary Benefits | Unum US
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, beginning of year	586.8		598.7		616.7
Policyholder Account Balance, Premium Received	58.5		64.5		69.6
Policyholder Account Balance, Policy Charge	(60.4)	[2]	(64.5)	[2]	(69.3)
Policyholder Account Balance, Surrender and Withdrawal	(33.3)		(32.9)		(39.5)
Policyholder Account Balance, Benefit Payment	(9.7)		(10.7)		(11.4)
Policyholder Account Balance, Interest Expense	22.2		23.5		23.7
Policyholder Account Balance, Increase (Decrease) from Other Change	14.5		8.2		8.9
Policyholder Account Balance, before Reserves in excess of Account Balance, end of period	578.6		586.8		598.7
Policyholder Account Balance, Reserves in excess of Account Balance	99.5		92.9		82.9
Policyholders' Account Balances	678.1		679.7		681.6
Policyholder Account Balance, Reinsurance recoverable	0.9		1.1		1.2
Policyholder Account Balance, After reinsurance recoverable	$ 677.2		$ 678.6		$ 680.4
Policyholder Account Balance, Weighted Average Crediting Rate	3.90%		4.00%		3.90%
Policyholder Account Balance, Net Amount at Risk	$ 4,495.6	[3]	$ 4,908.0	[3]	$ 5,356.1
Policyholder Account Balance, Cash Surrender Value	566.9		583.3		584.8
Other Insurance Product Line | Closed Block
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, beginning of year	4,159.4		4,231.7		4,218.0
Policyholder Account Balance, Premium Received	31.7		32.2		37.4
Policyholder Account Balance, Policy Charge	(107.2)	[2]	(105.5)	[2]	(101.2)
Policyholder Account Balance, Surrender and Withdrawal	(18.8)		(21.3)		(14.3)
Policyholder Account Balance, Benefit Payment	(216.2)		(292.8)		(234.1)
Policyholder Account Balance, Interest Expense	322.1		311.5		315.4
Policyholder Account Balance, Increase (Decrease) from Other Change	(88.3)		3.6		10.5
Policyholder Account Balance, before Reserves in excess of Account Balance, end of period	4,082.7		4,159.4		4,231.7
Policyholder Account Balance, Reserves in excess of Account Balance	37.1		32.0		30.8
Policyholders' Account Balances	4,119.8		4,191.4		4,262.5
Policyholder Account Balance, Reinsurance recoverable	4,119.8		4,191.4		4,262.5
Policyholder Account Balance, After reinsurance recoverable	$ 0.0		$ 0.0		$ 0.0
Policyholder Account Balance, Weighted Average Crediting Rate	8.00%		7.60%		7.80%
Policyholder Account Balance, Net Amount at Risk	$ 1,824.3	[3]	$ 1,931.6	[3]	$ 2,052.0
Policyholder Account Balance, Cash Surrender Value	$ 4,062.3		$ 4,045.9		$ 4,134.7

[1] 2We previously reported policy and contract benefits and reserves for future policy and contract benefits liabilities in our consolidated balance sheet as of 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.

[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.