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, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
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	$ 0.0	$ 0.0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.8	5.2	6.9	5.5
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	5.4	(8.4)	5.4	(8.4)	$ 4.6	$ (1.5)	$ (13.6)	$ (13.9)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	0.8	5.2	6.9	5.5
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
US States and Political Subdivisions Debt Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		0.0		0.0			0.1	0.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0.0		0.0
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		0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		(0.1)		(0.2)
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)		0.0		0.0
Public Utilities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		0.0		0.0			0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		0.2		0.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		(0.3)		(0.3)
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		(3.2)		(3.3)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		3.3		3.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		0.0		0.0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		(0.3)		(0.3)
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]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	46.5	24.1	46.5	24.1	38.7	32.9	28.8	22.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	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)	0.4	(0.1)	0.5	(0.1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	4.9	3.9	10.6	10.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities	(0.7)	(0.1)	(1.2)	(0.3)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	3.3	0.0	3.8	0.4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(0.1)	(8.4)	(0.1)	(8.8)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	0.4	(0.1)	0.5	(0.1)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0	0.0	0.0
All Other Corporate Bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	53.3	128.8	53.3	128.8	90.2	123.4	139.1	158.7
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0.0	(0.4)	(2.6)	(0.5)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	2.5	(29.0)	5.1	(24.1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0.0	30.0	2.3	31.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities	(26.7)	(92.4)	(154.2)	(152.1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	23.1	195.3	149.5	205.8
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(35.8)	(113.8)	(70.2)	(90.5)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	2.5	(29.0)	5.1	(24.1)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0	(2.6)	0.0
Fixed Maturity Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	99.8	152.9	99.8	152.9	128.9	156.3	168.0	180.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0.0	(0.2)	(2.6)	(0.2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	2.9	(29.4)	5.6	(24.5)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	4.9	33.9	12.9	42.4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities	(27.4)	(95.7)	(155.4)	(155.7)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	26.4	198.6	153.3	209.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(35.9)	(122.3)	(70.3)	(99.5)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	2.9	(29.4)	5.6	(24.5)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0	(2.6)	0.0
Equity Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	21.7	17.5	21.7	17.5	$ 21.6	$ 21.6	$ 16.2	$ 16.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0.1	0.5	0.1	0.5
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	0.0	0.8	0.0	0.8
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales/Maturities	0.0	0.0	0.0	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
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)	$ 0.1	$ 0.5	$ 0.1	$ 0.5