Policyholders' Account Balances (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024		Dec. 31, 2023		Dec. 31, 2022
Policyholder Account Balance [Roll Forward]
Policyholders' Account Balances	$ 5,633.7		$ 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		$ 5,679.6
Policyholder Account Balance, Premium Received	159.7		175.1		187.6
Policyholder Account Balance, Policy Charge	(230.1)	[1]	(242.3)	[1]	(248.0)
Policyholder Account Balance, Surrender and Withdrawal	(81.3)		(88.9)		(90.4)
Policyholder Account Balance, Benefit Payment	(258.9)		(233.1)		(311.5)
Policyholder Account Balance, Interest Expense	355.5		378.5		369.2
Policyholder Account Balance, Increase (Decrease) from Other Change	10.9		(73.7)		12.1
Policyholder Account Balance, before Reserves in excess of Account Balance, end of period	5,470.0		5,514.2		5,598.6
Policyholder Account Balance, Reserves in excess of Account Balance	163.7		153.5		141.6
Policyholders' Account Balances	5,633.7		5,667.7		5,740.2
Policyholder Account Balance, Reinsurance recoverable	4,096.3		4,120.7		4,192.7
Policyholder Account Balance, After reinsurance recoverable	$ 1,537.4		$ 1,547.0		$ 1,547.5
Policyholder Account Balance, Weighted Average Crediting Rate	6.70%		7.00%		6.70%
Policyholder Account Balance, Net Amount at Risk	$ 14,058.3	[2]	$ 15,080.0	[2]	$ 16,178.1
Policyholder Account Balance, Cash Surrender Value	5,410.1		5,442.7		5,430.1
Colonial Life
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, beginning of year	852.9		852.4		849.2
Policyholder Account Balance, Premium Received	79.5		84.9		90.9
Policyholder Account Balance, Policy Charge	(71.9)	[1]	(74.7)	[1]	(78.0)
Policyholder Account Balance, Surrender and Withdrawal	(38.0)		(36.8)		(36.2)
Policyholder Account Balance, Benefit Payment	(7.8)		(7.2)		(8.0)
Policyholder Account Balance, Interest Expense	34.1		34.2		34.2
Policyholder Account Balance, Increase (Decrease) from Other Change	0.2		0.1		0.3
Policyholder Account Balance, before Reserves in excess of Account Balance, end of period	849.0		852.9		852.4
Policyholder Account Balance, Reserves in excess of Account Balance	13.5		16.9		16.7
Policyholders' Account Balances	862.5		869.8		869.1
Policyholder Account Balance, Reinsurance recoverable	0.0		0.0		0.2
Policyholder Account Balance, After reinsurance recoverable	$ 862.5		$ 869.8		$ 868.9
Policyholder Account Balance, Weighted Average Crediting Rate	4.10%		4.10%		4.10%
Policyholder Account Balance, Net Amount at Risk	$ 8,201.1	[2]	$ 8,760.1	[2]	$ 9,338.5
Policyholder Account Balance, Cash Surrender Value	819.8		813.5		800.9
Voluntary Benefits | Unum US
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, beginning of year	578.6		586.8		598.7
Policyholder Account Balance, Premium Received	53.7		58.5		64.5
Policyholder Account Balance, Policy Charge	(56.8)	[1]	(60.4)	[1]	(64.5)
Policyholder Account Balance, Surrender and Withdrawal	(30.9)		(33.3)		(32.9)
Policyholder Account Balance, Benefit Payment	(6.0)		(9.7)		(10.7)
Policyholder Account Balance, Interest Expense	20.8		22.2		23.5
Policyholder Account Balance, Increase (Decrease) from Other Change	9.4		14.5		8.2
Policyholder Account Balance, before Reserves in excess of Account Balance, end of period	568.8		578.6		586.8
Policyholder Account Balance, Reserves in excess of Account Balance	106.9		99.5		92.9
Policyholders' Account Balances	675.7		678.1		679.7
Policyholder Account Balance, Reinsurance recoverable	0.8		0.9		1.1
Policyholder Account Balance, After reinsurance recoverable	$ 674.9		$ 677.2		$ 678.6
Policyholder Account Balance, Weighted Average Crediting Rate	3.70%		3.90%		4.00%
Policyholder Account Balance, Net Amount at Risk	$ 4,132.2	[2]	$ 4,495.6	[2]	$ 4,908.0
Policyholder Account Balance, Cash Surrender Value	559.0		566.9		583.3
Other Insurance Product Line | Closed Block
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, beginning of year	4,082.7		4,159.4		4,231.7
Policyholder Account Balance, Premium Received	26.5		31.7		32.2
Policyholder Account Balance, Policy Charge	(101.4)	[1]	(107.2)	[1]	(105.5)
Policyholder Account Balance, Surrender and Withdrawal	(12.4)		(18.8)		(21.3)
Policyholder Account Balance, Benefit Payment	(245.1)		(216.2)		(292.8)
Policyholder Account Balance, Interest Expense	300.6		322.1		311.5
Policyholder Account Balance, Increase (Decrease) from Other Change	1.3		(88.3)		3.6
Policyholder Account Balance, before Reserves in excess of Account Balance, end of period	4,052.2		4,082.7		4,159.4
Policyholder Account Balance, Reserves in excess of Account Balance	43.3		37.1		32.0
Policyholders' Account Balances	4,095.5		4,119.8		4,191.4
Policyholder Account Balance, Reinsurance recoverable	4,095.5		4,119.8		4,191.4
Policyholder Account Balance, After reinsurance recoverable	$ 0.0		$ 0.0		$ 0.0
Policyholder Account Balance, Weighted Average Crediting Rate	7.60%		8.00%		7.60%
Policyholder Account Balance, Net Amount at Risk	$ 1,725.0	[2]	$ 1,824.3	[2]	$ 1,931.6
Policyholder Account Balance, Cash Surrender Value	$ 4,031.3		$ 4,062.3		$ 4,045.9

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

[2] 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.