Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance	$ 17.8	$ 11.5
Derivative Liability Gain (Loss) Included in Earnings	(3.0)	(1.9)
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	14.8	9.6
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)	(3.0)	(1.9)
Public Utility, Bonds
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	0.0	(1.5)
Investment Gain (Loss) included in OCI	0.0	1.6
Investment Purchases	0.0	0.0
Investment Sales	(10.0)	(12.3)
Investment Level 3 Transfers Into	10.0	12.2
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	103.1	73.5
Investment Gain (Loss) included In Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	0.6	(0.6)
Investment Purchases	103.4	4.8
Investment Sales	(9.5)	(0.5)
Investment Level 3 Transfers Into	0.1	0.0
Investment Level 3 Transfers Out of	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	197.7	77.2
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in OCI	0.7	(0.6)
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in Earnings	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	63.9	71.5
Investment Gain (Loss) included In Earnings	0.0	(6.2)
Investment Gain (Loss) included in OCI	0.1	(8.3)
Investment Purchases	0.0	0.0
Investment Sales	(39.0)	(70.3)
Investment Level 3 Transfers Into	82.8	83.8
Investment Level 3 Transfers Out of	(47.4)	(38.2)
Assets Measured with Unobservable Inputs, Ending Balance	60.4	32.3
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in OCI	(0.4)	(8.3)
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	167.0	145.0
Investment Gain (Loss) included In Earnings	0.0	(7.7)
Investment Gain (Loss) included in OCI	0.7	(7.3)
Investment Purchases	103.4	4.8
Investment Sales	(58.5)	(83.1)
Investment Level 3 Transfers Into	92.9	96.0
Investment Level 3 Transfers Out of	(47.4)	(38.2)
Assets Measured with Unobservable Inputs, Ending Balance	258.1	109.5
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in OCI	0.3	(8.9)
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in Earnings	0.0	0.0
Perpetual Preferred and Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	19.8	24.4
Investment Gain (Loss) included In Earnings	0.0	0.7
Investment Gain (Loss) included in OCI	0.0	0.0
Investment Purchases	0.0	1.7
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	19.8	26.8
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	$ 0.7