Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance	$ (25.6)	$ (17.6)	$ (31.1)	$ (15.9)
Derivative Liability Gain (Loss) Included in Earnings	(0.8)	(2.3)	4.7	(4.0)
Derivative Liability Gain (Loss) Included in OCI	0.0	0.0	0.0	0.0
Derivative Purchases	0.0	0.0	0.0	0.0
Derivative Sales	0.0	0.0	0.0	0.0
Derivative Transfers Into Level 3	0.0	0.0	0.0	0.0
Derivative Transfers Out of Level 3	0.0	0.0	0.0	0.0
Derivatives Measured with Unobservable Inputs, Ending Balance	(26.4)	(19.9)	(26.4)	(19.9)
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance		35.7		0.0
Investment Gain (Loss) included in Earnings		0.0		0.0
Investment Gain (Loss) included in OCI		0.8		0.0
Investment Purchases		0.0		0.0
Investment Sales		0.0		(0.1)
Investment Level 3 Transfers Into		0.0		36.6
Investment Level 3 Transfers Out of		0.0		0.0
Assets Measured with Unobservable Inputs, Ending Balance		36.5		36.5
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	31.6	32.3	31.4	0.0
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	0.3	(0.8)	0.5	(1.3)
Investment Purchases	0.0	0.0	0.0	0.0
Investment Sales	0.0	0.0	0.0	0.0
Investment Level 3 Transfers Into	0.0	0.0	0.0	32.8
Investment Level 3 Transfers Out of	0.0	0.0	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	31.9	31.5	31.9	31.5
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	224.0	284.2	84.7	207.7
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	(2.0)	(0.8)	7.2	(3.6)
Investment Purchases	0.0	0.0	0.0	0.0
Investment Sales	0.0	0.0	(0.2)	0.0
Investment Level 3 Transfers Into	80.3	49.4	151.8	81.7
Investment Level 3 Transfers Out of	(113.2)	(163.6)	(54.4)	(116.6)
Assets Measured with Unobservable Inputs, Ending Balance	189.1	169.2	189.1	169.2
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance		0.5		0.0
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		0.5
Investment Level 3 Transfers Out of		0.0		0.0
Assets Measured with Unobservable Inputs, Ending Balance		0.5		0.5
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	496.0	987.2	1,495.8	1,150.1
Investment Gain (Loss) included in Earnings	0.0	1.9	0.0	1.8
Investment Gain (Loss) included in OCI	16.0	(16.1)	61.8	(35.4)
Investment Purchases	0.0	9.9	14.0	26.0
Investment Sales	(9.7)	(37.5)	(60.5)	(71.3)
Investment Level 3 Transfers Into	595.8	409.3	405.5	415.2
Investment Level 3 Transfers Out of	(84.0)	(538.4)	(902.5)	(670.1)
Assets Measured with Unobservable Inputs, Ending Balance	1,014.1	816.3	1,014.1	816.3
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	21.0	22.2	21.1	22.8
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	0.0	(0.6)	0.0	(1.2)
Investment Purchases	0.0	0.0	0.0	0.0
Investment Sales	0.0	0.0	0.0	0.0
Investment Level 3 Transfers Into	0.0	0.0	0.0	0.0
Investment Level 3 Transfers Out of	(21.0)	0.0	(21.1)	0.0
Assets Measured with Unobservable Inputs, Ending Balance	0.0	21.6	0.0	21.6
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	772.6	1,362.1	1,633.0	1,380.6
Investment Gain (Loss) included in Earnings	0.0	1.9	0.0	1.8
Investment Gain (Loss) included in OCI	14.3	(17.5)	69.5	(41.5)
Investment Purchases	0.0	9.9	14.0	26.0
Investment Sales	(9.7)	(37.5)	(60.7)	(71.4)
Investment Level 3 Transfers Into	676.1	458.7	557.3	566.8
Investment Level 3 Transfers Out of	(218.2)	(702.0)	(978.0)	(786.7)
Assets Measured with Unobservable Inputs, Ending Balance	1,235.1	1,075.6	1,235.1	1,075.6
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	4.6	1.1	4.6	1.1
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	0.0	0.0	0.0	0.0
Investment Purchases	0.0	0.0	0.0	0.0
Investment Sales	0.0	0.0	0.0	0.0
Investment Level 3 Transfers Into	0.0	0.0	0.0	0.0
Investment Level 3 Transfers Out of	0.0	0.0	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	$ 4.6	$ 1.1	$ 4.6	$ 1.1

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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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).

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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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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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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.

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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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