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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance	$ (19.9)	$ (31.9)	$ (15.9)	$ (46.7)
Derivative Liability Gain (Loss) Included in Earnings	6.1	6.7	2.1	21.5
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	(13.8)	(25.2)	(13.8)	(25.2)
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	36.5	36.8	0.0	89.5
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	(1.5)	(1.1)	(1.6)	(0.9)
Investment Purchases	0.0	0.0	0.0	0.0
Investment Sales	0.0	0.0	(0.4)	0.0
Investment Level 3 Transfers Into	28.9	52.9	65.9	0.0
Investment Level 3 Transfers Out of	0.0	0.0	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	63.9	88.6	63.9	88.6
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	31.5		0.0
Investment Gain (Loss) included in Earnings	0.0		0.0
Investment Gain (Loss) included in OCI	0.1		(1.2)
Investment Purchases	0.0		0.0
Investment Sales	0.0		0.0
Investment Level 3 Transfers Into	0.0		32.8
Investment Level 3 Transfers Out of	0.0		0.0
Assets Measured with Unobservable Inputs, Ending Balance	31.6		31.6
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	169.2	192.5	207.7	265.3
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	(1.2)	0.0	(6.7)	0.2
Investment Purchases	0.0	0.0	0.0	8.0
Investment Sales	0.0	0.0	0.0	(4.8)
Investment Level 3 Transfers Into	66.9	133.8	120.7	78.0
Investment Level 3 Transfers Out of	(112.5)	(118.2)	(199.3)	(138.6)
Assets Measured with Unobservable Inputs, Ending Balance	122.4	208.1	122.4	208.1
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	816.3	853.2	1,150.1	1,459.7
Investment Gain (Loss) included in Earnings	0.0	0.0	5.8	(0.6)
Investment Gain (Loss) included in OCI	(6.9)	14.3	(49.5)	25.1
Investment Purchases	0.0	38.9	47.0	88.9
Investment Sales	(21.0)	(33.2)	(117.7)	(131.8)
Investment Level 3 Transfers Into	358.8	390.0	471.0	438.0
Investment Level 3 Transfers Out of	(255.4)	(169.9)	(614.9)	(786.0)
Assets Measured with Unobservable Inputs, Ending Balance	891.8	1,093.3	891.8	1,093.3
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	21.6	23.1	22.8	23.2
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	(0.1)	(0.1)	(1.3)	(0.2)
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	0.0	0.0	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	21.5	23.0	21.5	23.0
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1,075.6	1,105.6	1,380.6	1,837.7
Investment Gain (Loss) included in Earnings	0.0	0.0	5.8	(0.6)
Investment Gain (Loss) included in OCI	(9.6)	13.1	(60.3)	24.2
Investment Purchases	0.0	38.9	47.0	96.9
Investment Sales	(21.0)	(33.2)	(118.1)	(136.6)
Investment Level 3 Transfers Into	454.6	576.7	690.9	516.0
Investment Level 3 Transfers Out of	(367.9)	(288.1)	(814.2)	(924.6)
Assets Measured with Unobservable Inputs, Ending Balance	1,131.7	1,413.0	1,131.7	1,413.0
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1.1	1.2	1.1	1.2
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	0.0	0.1	0.0	0.1
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	$ 1.1	$ 1.3	$ 1.1	$ 1.3

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 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 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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