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, 2024	Dec. 31, 2023
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance	$ (1.5)	$ (13.9)
Derivative Liability Gain (Loss) Included in Earnings	13.0	12.4
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	11.5	(1.5)
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)	13.0	12.4
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	0.0	0.2
Investment Gain (Loss) included in Earnings		0.0
Investment Gain (Loss) included in OCI		0.0
Investment Purchases		0.0
Investment Sales		0.0
Investment Level 3 Transfers Into		0.0
Investment Level 3 Transfers Out of		0.2
Assets Measured with Unobservable Inputs, Ending Balance		0.0
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in OCI		0.0
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in Earnings		0.0
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	0.0	0.0
Investment Gain (Loss) included in Earnings	(1.1)	0.1
Investment Gain (Loss) included in OCI	1.3	(0.1)
Investment Purchases	0.0	0.0
Investment Sales	(20.4)	(50.5)
Investment Level 3 Transfers Into	20.2	50.5
Investment Level 3 Transfers Out of	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	0.0	0.0
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in OCI	0.0	0.0
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in Earnings	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	32.9	22.0
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	0.6	(0.2)
Investment Purchases	38.5	20.0
Investment Sales	(2.6)	(0.5)
Investment Level 3 Transfers Into	4.1	0.4
Investment Level 3 Transfers Out of	0.0	8.8
Assets Measured with Unobservable Inputs, Ending Balance	73.5	32.9
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in OCI	(0.6)	0.2
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in Earnings		0.0
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	123.4	158.7
Investment Gain (Loss) included in Earnings	(3.5)	(5.0)
Investment Gain (Loss) included in OCI	8.9	(15.8)
Investment Purchases	3.2	1.5
Investment Sales	(329.8)	(334.1)
Investment Level 3 Transfers Into	318.1	385.0
Investment Level 3 Transfers Out of	48.8	66.9
Assets Measured with Unobservable Inputs, Ending Balance	71.5	123.4
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in OCI	(8.9)	15.8
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in Earnings	0.0	0.0
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	156.3	180.9
Investment Gain (Loss) included in Earnings	(4.6)	(4.9)
Investment Gain (Loss) included in OCI	10.8	(16.1)
Investment Purchases	41.7	21.5
Investment Sales	(352.8)	(385.1)
Investment Level 3 Transfers Into	342.4	435.9
Investment Level 3 Transfers Out of	48.8	75.9
Assets Measured with Unobservable Inputs, Ending Balance	145.0	156.3
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in OCI	(9.5)	16.0
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in Earnings	0.0	0.0
Perpetual Preferred Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	21.6	16.2
Investment Gain (Loss) included in Earnings	2.3	0.6
Investment Gain (Loss) included in OCI	0.0	0.0
Investment Purchases	0.5	4.8
Investment Sales	0.0	0.0
Investment Level 3 Transfers Into	0.0	0.0
Investment Level 3 Transfers Out of	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	24.4	21.6
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in OCI	0.0	0.0
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in Earnings	$ 2.3	$ 0.6