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, 2022	Dec. 31, 2021
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance	$ (30.1)	$ (39.8)
Derivative Liability Gain (Loss) Included in Earnings	16.2	9.7
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	(13.9)	(30.1)
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)	16.2	9.7
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	13.4	15.5
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(0.1)	(2.1)
Investment Purchases	0.0	0.0
Investment Sales	0.0	0.0
Investment Level 3 Transfers Into	0.3	0.0
Investment Level 3 Transfers Out of	13.4	0.0
Assets Measured with Unobservable Inputs, Ending Balance	0.2	13.4
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	0.1	2.1
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	20.8	21.8
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(0.8)	(1.0)
Investment Purchases	0.0	0.0
Investment Sales	(20.0)	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	0.0	20.8
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	0.8	1.0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	44.5	185.7
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(2.8)	(2.3)
Investment Purchases	0.0	0.0
Investment Sales	(12.8)	(44.0)
Investment Level 3 Transfers Into	15.6	36.0
Investment Level 3 Transfers Out of	44.5	130.9
Assets Measured with Unobservable Inputs, Ending Balance	0.0	44.5
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	2.8	2.3
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	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	187.2	81.3
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(0.3)	(96.0)
Investment Purchases	20.7	0.0
Investment Sales	(10.6)	(72.3)
Investment Level 3 Transfers Into	0.0	274.2
Investment Level 3 Transfers Out of	175.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	22.0	187.2
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	0.3	96.0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	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	861.5	943.1
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(32.0)	(24.6)
Investment Purchases	25.3	249.9
Investment Sales	(102.3)	(80.2)
Investment Level 3 Transfers Into	161.9	77.6
Investment Level 3 Transfers Out of	755.7	304.3
Assets Measured with Unobservable Inputs, Ending Balance	158.7	861.5
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	32.0	24.6
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1,127.4	1,247.4
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(36.0)	(126.0)
Investment Purchases	46.0	249.9
Investment Sales	(145.7)	(196.5)
Investment Level 3 Transfers Into	177.8	387.8
Investment Level 3 Transfers Out of	988.6	435.2
Assets Measured with Unobservable Inputs, Ending Balance	180.9	1,127.4
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	36.0	126.0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
Perpetual Preferred Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	5.8	4.7
Investment Gain (Loss) included in Earnings	3.0	0.1
Investment Gain (Loss) included in OCI	0.0	0.0
Investment Purchases	7.1	1.0
Investment Sales	0.0	0.0
Investment Level 3 Transfers Into	0.3	0.0
Investment Level 3 Transfers Out of	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	16.2	5.8
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)	$ 3.0	$ 0.1

Amount of unrealized gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.

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Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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No definition available.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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No definition available.

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Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

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Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Amount of unrealized gain (loss) recognized in income for derivative asset (liability) after deduction of derivative liability (asset), measured at fair value using unobservable input (level 3) and still held.

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Amount of gain (loss) recognized in income from derivative asset (liability) after deduction of derivative liability (asset), measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Amount of gain (loss) recognized in other comprehensive income from derivative asset (liability) after deduction of derivative liability (asset), measured at fair value on recurring basis using unobservable input (level 3).

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 820
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Amount of purchases of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Amount of sales of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfers of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset) into level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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Amount of transfers of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset) out of level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of unrealized gain (loss) recognized in other comprehensive income (OCI) from derivative asset (liability) after deduction of derivative liability (asset), measured at fair value using unobservable input (level 3) and still held.

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