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		6 Months Ended
		Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
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)				(0.9)	6.9
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance		$ 9.6	$ 4.6	11.5	(1.5)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings		1.0	0.8	(0.9)	6.9
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Other Comprehensive Income (Loss)		0.0	0.0	0.0	0.0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0.0	0.0	0.0	0.0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Sales		0.0	0.0	0.0	0.0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers into Level 3		0.0	0.0	0.0	0.0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3		0.0	0.0	0.0	0.0
Derivatives Measured with Unobservable Inputs, Ending Balance		10.6	5.4	10.6	5.4
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)		1.0	0.8
Public Utilities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets Measured with Unobservable Inputs, Beginning Balance		0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		0.0		(1.5)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0.0		1.7
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities		(9.0)		(21.3)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		9.0		21.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		0.0		0.0
Assets Measured with Unobservable Inputs, Ending Balance		0.0		0.0
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)		0.0		0.0
Mortgage/Asset-backed Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets Measured with Unobservable Inputs, Beginning Balance	[1]	77.2	38.7	73.5	32.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	[1]	0.0	0.0	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	[1]	0.9	0.4	0.3	0.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	[1]	7.1	4.9	11.9	10.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities	[1]	(3.7)	(0.7)	(4.2)	(1.2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	[1]	0.0	3.3	0.0	3.8
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	[1]	(0.1)	(0.1)	(0.1)	(0.1)
Assets Measured with Unobservable Inputs, Ending Balance	[1]	81.4	46.5	81.4	46.5
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	[1]	0.9	0.4	0.3	0.5
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	[1]	0.0	0.0	0.0	0.0
All Other Corporate Bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets Measured with Unobservable Inputs, Beginning Balance		32.3	90.2	71.5	123.4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		0.0	0.0	(6.2)	(2.6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		(0.1)	2.5	(8.3)	5.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		1.0	0.0	1.0	2.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities		(29.3)	(26.7)	(99.6)	(154.2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		16.0	23.1	99.7	149.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		0.0	(35.8)	(38.2)	(70.2)
Assets Measured with Unobservable Inputs, Ending Balance		19.9	53.3	19.9	53.3
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		(0.1)	2.5	(8.3)	5.1
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)		0.0	0.0	0.0	(2.6)
Fixed Maturity Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets Measured with Unobservable Inputs, Beginning Balance		109.5	128.9	145.0	156.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		0.0	0.0	(7.7)	(2.6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0.8	2.9	(6.3)	5.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		8.1	4.9	12.9	12.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities		(42.0)	(27.4)	(125.1)	(155.4)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		25.0	26.4	120.8	153.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		(0.1)	(35.9)	(38.3)	(70.3)
Assets Measured with Unobservable Inputs, Ending Balance		101.3	99.8	101.3	99.8
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		0.8	2.9	(8.0)	5.6
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)		0.0	0.0	0.0	(2.6)
Perpetual Preferred and Equity Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assets Measured with Unobservable Inputs, Beginning Balance		26.8	21.6	24.4	21.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		6.9	0.1	7.6	0.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0.0	0.0	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		3.0	0.0	4.7	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities		(12.9)	0.0	(12.9)	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		0.0	0.0	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		0.0	0.0	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance		23.8	21.7	23.8	21.7
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		0.0	0.0	0.0	0.0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)		$ 0.0	$ 0.1	$ 0.7	$ 0.1

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