Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	$ 122.2	$ 140.1
Investment Gain (Loss) included in Earnings	0.0	(0.1)
Investment Gain (Loss) included in OCI	1.9	(5.3)
Investment Purchases	0.0	12.0
Investment Sales	(0.5)	(16.5)
Level 3 Transfers Into	0.0	0.0
Level 3 Transfers Out of	(34.1)	(8.0)
Ending Balance of Assets Measured with Unobservable Inputs	89.5	122.2
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	52.9	69.3
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	0.0	(1.4)
Investment Purchases	0.0	0.0
Investment Sales	0.0	(15.0)
Level 3 Transfers Into	0.0	0.0
Level 3 Transfers Out of	(52.9)	0.0
Ending Balance of Assets Measured with Unobservable Inputs	0.0	52.9
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	274.1	315.0
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(0.8)	(6.9)
Investment Purchases	0.0	40.0
Investment Sales	(3.7)	(2.4)
Level 3 Transfers Into	157.6	118.5
Level 3 Transfers Out of	(161.9)	(190.1)
Ending Balance of Assets Measured with Unobservable Inputs	265.3	274.1
Corporate Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	1,408.2	1,425.3
Investment Gain (Loss) included in Earnings	1.4	(7.0)
Investment Gain (Loss) included in OCI	6.9	(113.2)
Investment Purchases	115.9	55.3
Investment Sales	(111.2)	(220.8)
Level 3 Transfers Into	792.8	810.2
Level 3 Transfers Out of	(754.3)	(541.6)
Ending Balance of Assets Measured with Unobservable Inputs	1,459.7	1,408.2
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	23.8	24.9
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(0.6)	(1.1)
Investment Purchases	0.0	0.0
Investment Sales	0.0	0.0
Level 3 Transfers Into	0.0	0.0
Level 3 Transfers Out of	0.0	0.0
Ending Balance of Assets Measured with Unobservable Inputs	23.2	23.8
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	1,881.2	1,974.6
Investment Gain (Loss) included in Earnings	1.4	(7.1)
Investment Gain (Loss) included in OCI	7.4	(127.9)
Investment Purchases	115.9	107.3
Investment Sales	(115.4)	(254.7)
Level 3 Transfers Into	950.4	928.7
Level 3 Transfers Out of	(1,003.2)	(739.7)
Ending Balance of Assets Measured with Unobservable Inputs	1,837.7	1,881.2
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	1.4	1.4
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	0.1	0.0
Investment Purchases	0.0	0.0
Investment Sales	(0.3)	0.0
Level 3 Transfers Into	0.0	0.0
Level 3 Transfers Out of	0.0	0.0
Ending Balance of Assets Measured with Unobservable Inputs	1.2	1.4
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Derivatives Measured with Unobservable Inputs, Beginning Balance	(87.6)	(49.9)
Derivative Liability Gain (Loss) Included in Earnings	40.9	(37.7)
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	$ (46.7)	$ (87.6)

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 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 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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