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, 2025	Dec. 31, 2024
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance		$ 11.5	$ (1.5)
Derivative Liability Gain (Loss) Included in Earnings		6.3	13.0
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		17.8	11.5
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)		6.3	13.0
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		(1.5)	(1.1)
Investment Gain (Loss) included in OCI		1.9	1.3
Investment Purchases		0.0	0.0
Investment Sales		(43.7)	(20.4)
Investment Level 3 Transfers Into		43.3	20.2
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	[1]	73.5	32.9
Investment Gain (Loss) included in Earnings	[1]	0.0	0.0
Investment Gain (Loss) included in OCI	[1]	0.3	0.6
Investment Purchases	[1]	38.0	38.5
Investment Sales	[1]	(8.6)	(2.6)
Investment Level 3 Transfers Into	[1]	0.0	4.1
Investment Level 3 Transfers Out of	[1]	0.1	0.0
Assets Measured with Unobservable Inputs, Ending Balance	[1]	103.1	73.5
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in OCI	[1]	(0.3)	(0.6)
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in Earnings	[1]	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		71.5	123.4
Investment Gain (Loss) included in Earnings		(6.2)	(3.5)
Investment Gain (Loss) included in OCI		9.1	8.9
Investment Purchases		21.8	3.2
Investment Sales		(268.6)	(329.8)
Investment Level 3 Transfers Into		274.6	318.1
Investment Level 3 Transfers Out of		38.3	48.8
Assets Measured with Unobservable Inputs, Ending Balance		63.9	71.5
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in OCI		(7.7)	(8.9)
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		145.0	156.3
Investment Gain (Loss) included in Earnings		(7.7)	(4.6)
Investment Gain (Loss) included in OCI		11.3	10.8
Investment Purchases		59.8	41.7
Investment Sales		(320.9)	(352.8)
Investment Level 3 Transfers Into		317.9	342.4
Investment Level 3 Transfers Out of		38.4	48.8
Assets Measured with Unobservable Inputs, Ending Balance		167.0	145.0
Change in Unrealized Gain (Loss) on Securities Held at the End of Period Included in OCI		(8.0)	(9.5)
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		24.4	21.6
Investment Gain (Loss) included in Earnings		13.5	2.3
Investment Gain (Loss) included in OCI		0.0	0.0
Investment Purchases		4.7	0.5
Investment Sales		(22.8)	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	24.4
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		$ 0.8	$ 2.3

[1] Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types.