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, 2020	Mar. 31, 2019
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance	$ (22.8)	$ (31.1)
Derivative Liability Gain (Loss) Included in Earnings	(86.9)	5.5
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	(109.7)	(25.6)
US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	41.8
Investment Gain (Loss) included in Earnings	0.0
Investment Gain (Loss) included in OCI	2.8
Investment Purchases	0.0
Investment Sales	0.0
Investment Level 3 Transfers Into	0.0
Investment Level 3 Transfers Out of	(13.3)
Assets Measured with Unobservable Inputs, Ending Balance	31.3
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	2.9
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	21.8	31.4
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(0.9)	0.2
Investment Purchases	0.0	0.0
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	20.9	31.6
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	(0.9)
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	14.6	84.7
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(0.3)	7.9
Investment Purchases	0.0	0.0
Investment Sales	0.0	(0.4)
Investment Level 3 Transfers Into	18.0	208.6
Investment Level 3 Transfers Out of	(6.0)	(76.8)
Assets Measured with Unobservable Inputs, Ending Balance	26.3	224.0
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	0.0
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	34.1
Investment Gain (Loss) included in Earnings	0.0
Investment Gain (Loss) included in OCI	(0.8)
Investment Purchases	0.0
Investment Sales	0.0
Investment Level 3 Transfers Into	49.2
Investment Level 3 Transfers Out of	(7.4)
Assets Measured with Unobservable Inputs, Ending Balance	75.1
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	(0.2)
Corporate Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	600.5	1,495.8
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(50.6)	13.3
Investment Purchases	20.9	0.0
Investment Sales	0.0	(29.4)
Investment Level 3 Transfers Into	236.2	36.1
Investment Level 3 Transfers Out of	(254.0)	(1,019.8)
Assets Measured with Unobservable Inputs, Ending Balance	553.0	496.0
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	(2.2)
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance		21.1
Investment Gain (Loss) included in Earnings		0.0
Investment Gain (Loss) included in OCI		(0.1)
Investment Purchases		0.0
Investment Sales		0.0
Investment Level 3 Transfers Into		0.0
Investment Level 3 Transfers Out of		0.0
Assets Measured with Unobservable Inputs, Ending Balance		21.0
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	712.8	1,633.0
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(49.8)	21.3
Investment Purchases	20.9	0.0
Investment Sales	0.0	(29.8)
Investment Level 3 Transfers Into	303.4	244.7
Investment Level 3 Transfers Out of	(280.7)	(1,096.6)
Assets Measured with Unobservable Inputs, Ending Balance	706.6	772.6
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI	(0.4)
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	4.6	4.6
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	0.0	0.0
Investment Purchases	0.0	0.0
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	$ 4.6	$ 4.6

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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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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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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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 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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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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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
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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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