Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance	$ (13.9)	$ (30.1)
Derivative Liability Gain (Loss) Included in Earnings	12.4	16.2
Derivative Liability Gain (Loss) Included in OCI	0.0	0.0
Derivative Purchases	0.0	0.0
Derivative Sales	0.0	0.0
Derivative Transfers Into Level 3	0.0	0.0
Derivative Transfers Out of Level 3	0.0	0.0
Derivatives Measured with Unobservable Inputs, Ending Balance	(1.5)	(13.9)
Fair Value, Asset (Liability), Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	0.0	0.0
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Still Held, Unrealized Gain (Loss)	12.4	16.2
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	0.2	13.4
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	0.0	(0.1)
Investment Purchases	0.0	0.0
Investment Sales	0.0	0.0
Investment Level 3 Transfers Into	0.0	0.3
Investment Level 3 Transfers Out of	0.2	13.4
Assets Measured with Unobservable Inputs, Ending Balance	0.0	0.2
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	0.0	0.1
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	0.0	20.8
Investment Gain (Loss) included in Earnings		0.0
Investment Gain (Loss) included in OCI		(0.8)
Investment Purchases		0.0
Investment Sales		(20.0)
Investment Level 3 Transfers Into		0.0
Investment Level 3 Transfers Out of		0.0
Assets Measured with Unobservable Inputs, Ending Balance		0.0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		0.8
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)		0.0
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	0.0	44.5
Investment Gain (Loss) included in Earnings	0.1	0.0
Investment Gain (Loss) included in OCI	(0.1)	(2.8)
Investment Purchases	0.0	0.0
Investment Sales	(50.5)	(12.8)
Investment Level 3 Transfers Into	50.5	15.6
Investment Level 3 Transfers Out of	0.0	44.5
Assets Measured with Unobservable Inputs, Ending Balance	0.0	0.0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	0.0	2.8
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	22.0	187.2
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(0.2)	(0.3)
Investment Purchases	20.0	20.7
Investment Sales	(0.5)	(10.6)
Investment Level 3 Transfers Into	0.4	0.0
Investment Level 3 Transfers Out of	8.8	175.0
Assets Measured with Unobservable Inputs, Ending Balance	32.9	22.0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	0.2	0.3
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	158.7	861.5
Investment Gain (Loss) included in Earnings	(5.0)	0.0
Investment Gain (Loss) included in OCI	(15.8)	(32.0)
Investment Purchases	1.5	25.3
Investment Sales	(334.1)	(102.3)
Investment Level 3 Transfers Into	385.0	161.9
Investment Level 3 Transfers Out of	66.9	755.7
Assets Measured with Unobservable Inputs, Ending Balance	123.4	158.7
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	15.8	32.0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	180.9	1,127.4
Investment Gain (Loss) included in Earnings	(4.9)	0.0
Investment Gain (Loss) included in OCI	(16.1)	(36.0)
Investment Purchases	21.5	46.0
Investment Sales	(385.1)	(145.7)
Investment Level 3 Transfers Into	435.9	177.8
Investment Level 3 Transfers Out of	75.9	988.6
Assets Measured with Unobservable Inputs, Ending Balance	156.3	180.9
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	16.0	36.0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
Perpetual Preferred Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	16.2	5.8
Investment Gain (Loss) included in Earnings	0.6	3.0
Investment Gain (Loss) included in OCI	0.0	0.0
Investment Purchases	4.8	7.1
Investment Sales	0.0	0.0
Investment Level 3 Transfers Into	0.0	0.3
Investment Level 3 Transfers Out of	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	21.6	16.2
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	0.0	0.0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	$ 0.6	$ 3.0