Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Level 3) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
States, Municipalities, and Political Subdivisions
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	$ 140.1	$ 175.1
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	4.1	6.5
Investment Purchases	0	0
Investment Sales	0	0
Investment Level 3 Transfers Into	0	0
Investment Level 3 Transfers Out of	0	(25.5)
Assets Measured with Unobservable Inputs, Ending Balance	144.2	156.1
Foreign Governments
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	69.3	78.5
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	(0.2)	1.9
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	69.1	80.4
Public Utilities
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	315.0	139.3
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	3.1	3.7
Investment Purchases	0	0
Investment Sales	(0.1)	0
Investment Level 3 Transfers Into	300.8	77.8
Investment Level 3 Transfers Out of	(69.0)	(30.2)
Assets Measured with Unobservable Inputs, Ending Balance	549.8	190.6
Mortgage/Asset-backed Securities
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance		0.5
Investment Gain (Loss) included in Earnings		0
Investment Gain (Loss) included in OCI		0
Investment Purchases		0
Investment Sales		(0.1)
Investment Level 3 Transfers Into		0
Investment Level 3 Transfers Out of		0
Assets Measured with Unobservable Inputs, Ending Balance		0.4
Corporate Debt Securities
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1,425.3	1,923.3
Investment Gain (Loss) included in Earnings	(0.3)	0
Investment Gain (Loss) included in OCI	12.5	26.1
Investment Purchases	0	79.6
Investment Sales	(15.9)	(29.6)
Investment Level 3 Transfers Into	498.5	409.6
Investment Level 3 Transfers Out of	(284.3)	(816.0)
Assets Measured with Unobservable Inputs, Ending Balance	1,635.8	1,593.0
Redeemable Preferred Stocks
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	24.9	23.8
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	0	0.6
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	24.9	24.4
Fixed Maturity Securities
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1,974.6	2,340.5
Investment Gain (Loss) included in Earnings	(0.3)	0
Investment Gain (Loss) included in OCI	19.5	38.8
Investment Purchases	0	79.6
Investment Sales	(16.0)	(29.7)
Investment Level 3 Transfers Into	799.3	487.4
Investment Level 3 Transfers Out of	(353.3)	(871.7)
Assets Measured with Unobservable Inputs, Ending Balance	2,423.8	2,044.9
Equity Securities
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1.4	4.6
Investment Gain (Loss) included in Earnings	0	2.1
Investment Gain (Loss) included in OCI	0	(0.1)
Investment Purchases	0	0
Investment Sales	0	(3.5)
Investment Level 3 Transfers Into	0	0
Investment Level 3 Transfers Out of	0	0
Assets Measured with Unobservable Inputs, Ending Balance	1.4	3.1
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance	(49.9)	(53.2)
Derivative Liability Gain (Loss) Included in Earnings	(3.9)	8.5
Derivative Liability Gain (Loss) Included in OCI	0	0
Derivative Purchases	0	0
Derivative Sales	0	0
Derivative Transfers into Level 3	0	0
Derivative Transfers out of Level 3	0	0
Derivatives Measured with Unobservable Inputs, Ending Balance	$ (53.8)	$ (44.7)

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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 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 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 gain (loss) recognized in other comprehensive income (loss) 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 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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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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