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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	$ 61.6	$ 136.7	$ 122.2	$ 140.1
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	0.1	(2.4)	4.2	(4.7)
Investment Purchases	0.0	0.0	0.0	0.0
Investment Sales	0.0	0.0	(0.3)	(0.8)
Investment Level 3 Transfers Into	30.4	0.0	0.0	0.0
Investment Level 3 Transfers Out of	0.0	(7.7)	(34.0)	(8.0)
Assets Measured with Unobservable Inputs, Ending Balance	92.1	126.6	92.1	126.6
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance		69.8	52.9	69.3
Investment Gain (Loss) included in Earnings		0.0	0.0	0.0
Investment Gain (Loss) included in OCI		(1.5)	0.0	(1.0)
Investment Purchases		0.0	0.0	0.0
Investment Sales		0.0	0.0	0.0
Investment Level 3 Transfers Into		0.0	0.0	0.0
Investment Level 3 Transfers Out of		0.0	(52.9)	0.0
Assets Measured with Unobservable Inputs, Ending Balance	0.0	68.3	0.0	68.3
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	453.5	357.8	274.1	315.0
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	(3.2)	1.3	3.9	(2.9)
Investment Purchases	0.0	40.0	0.0	40.0
Investment Sales	(0.3)	0.0	(0.7)	(2.3)
Investment Level 3 Transfers Into	62.1	9.0	276.9	70.7
Investment Level 3 Transfers Out of	(107.8)	(88.6)	(149.9)	(101.0)
Assets Measured with Unobservable Inputs, Ending Balance	404.3	319.5	404.3	319.5
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1,359.6	1,321.9	1,408.2	1,425.3
Investment Gain (Loss) included in Earnings	0.8	0.0	1.4	2.1
Investment Gain (Loss) included in OCI	8.0	(13.9)	52.7	(35.8)
Investment Purchases	0.0	0.0	20.0	25.0
Investment Sales	(24.0)	(61.9)	(61.8)	(142.2)
Investment Level 3 Transfers Into	326.2	225.1	381.4	307.1
Investment Level 3 Transfers Out of	(697.7)	(456.1)	(829.0)	(566.4)
Assets Measured with Unobservable Inputs, Ending Balance	972.9	1,015.1	972.9	1,015.1
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	23.6	24.8	23.8	24.9
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	(0.1)	0.0	(0.3)	(0.1)
Investment Purchases	0.0	0.0	0.0	0.0
Investment Sales		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	23.5	24.8	23.5	24.8
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1,898.3	1,911.0	1,881.2	1,974.6
Investment Gain (Loss) included in Earnings	0.8	0.0	1.4	2.1
Investment Gain (Loss) included in OCI	4.8	(16.5)	60.5	(44.5)
Investment Purchases	0.0	40.0	20.0	65.0
Investment Sales	(24.3)	(61.9)	(62.8)	(145.3)
Investment Level 3 Transfers Into	418.7	234.1	658.3	377.8
Investment Level 3 Transfers Out of	(805.5)	(552.4)	(1,065.8)	(675.4)
Assets Measured with Unobservable Inputs, Ending Balance	1,492.8	1,554.3	1,492.8	1,554.3
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1.2	1.4	1.4	1.4
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	0.0	0.1	0.1	0.1
Investment Purchases	0.0	0.0	0.0	0.0
Investment Sales	0.0	0.0	(0.3)	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	1.2	1.5	1.2	1.5
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance	(83.0)	(55.8)	(87.6)	(49.9)
Derivative Liability Gain (Loss) Included in Earnings	9.7	(27.3)	14.3	(33.2)
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	$ (73.3)	$ (83.1)	$ (73.3)	$ (83.1)

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

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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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 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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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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