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	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance	$ (31.9)	$ (83.0)	$ (46.7)	$ (87.6)
Derivative Liability Gain (Loss) Included in Earnings	6.7	9.7	21.5	14.3
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	(25.2)	(73.3)	(25.2)	(73.3)
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	36.8	61.6	89.5	122.2
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	(1.1)	0.1	(0.9)	4.2
Investment Purchases	0.0	0.0	0.0	0.0
Investment Sales	0.0	0.0	0.0	(0.3)
Investment Level 3 Transfers Into	52.9	30.4	0.0	0.0
Investment Level 3 Transfers Out of	0.0	0.0	0.0	(34.0)
Assets Measured with Unobservable Inputs, Ending Balance	88.6	92.1	88.6	92.1
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance				52.9
Investment Gain (Loss) included in Earnings				0.0
Investment Gain (Loss) included in OCI				0.0
Investment Purchases				0.0
Investment Sales				0.0
Investment Level 3 Transfers Into				0.0
Investment Level 3 Transfers Out of				(52.9)
Assets Measured with Unobservable Inputs, Ending Balance		0.0		0.0
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	192.5	453.5	265.3	274.1
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	0.0	(3.2)	0.2	3.9
Investment Purchases	0.0	0.0	8.0	0.0
Investment Sales	0.0	(0.3)	(4.8)	(0.7)
Investment Level 3 Transfers Into	133.8	62.1	78.0	276.9
Investment Level 3 Transfers Out of	(118.2)	(107.8)	(138.6)	(149.9)
Assets Measured with Unobservable Inputs, Ending Balance	208.1	404.3	208.1	404.3
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	853.2	1,359.6	1,459.7	1,408.2
Investment Gain (Loss) included in Earnings	0.0	0.8	(0.6)	1.4
Investment Gain (Loss) included in OCI	14.3	8.0	25.1	52.7
Investment Purchases	38.9	0.0	88.9	20.0
Investment Sales	(33.2)	(24.0)	(131.8)	(61.8)
Investment Level 3 Transfers Into	390.0	326.2	438.0	381.4
Investment Level 3 Transfers Out of	(169.9)	(697.7)	(786.0)	(829.0)
Assets Measured with Unobservable Inputs, Ending Balance	1,093.3	972.9	1,093.3	972.9
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	23.1	23.6	23.2	23.8
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	(0.1)	(0.1)	(0.2)	(0.3)
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	0.0
Investment Level 3 Transfers Out of	0.0	0.0	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	23.0	23.5	23.0	23.5
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1,105.6	1,898.3	1,837.7	1,881.2
Investment Gain (Loss) included in Earnings	0.0	0.8	(0.6)	1.4
Investment Gain (Loss) included in OCI	13.1	4.8	24.2	60.5
Investment Purchases	38.9	0.0	96.9	20.0
Investment Sales	(33.2)	(24.3)	(136.6)	(62.8)
Investment Level 3 Transfers Into	576.7	418.7	516.0	658.3
Investment Level 3 Transfers Out of	(288.1)	(805.5)	(924.6)	(1,065.8)
Assets Measured with Unobservable Inputs, Ending Balance	1,413.0	1,492.8	1,413.0	1,492.8
Equity Securities - Private
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1.2	1.2	1.2	1.4
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	0.1	0.0	0.1	0.1
Investment Purchases	0.0	0.0	0.0	0.0
Investment Sales	0.0	0.0	0.0	(0.3)
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	$ 1.3	$ 1.2	$ 1.3	$ 1.2

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 the income statement for 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 other comprehensive income (loss) for 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 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 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 the income statement 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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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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