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, 2017	Dec. 31, 2016
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Derivatives Measured with Unobservable Inputs, Beginning Balance	$ (46.7)	$ (87.6)
Derivative Liability Gain (Loss) Included in Earnings	30.8	40.9
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	(15.9)	(46.7)
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	89.5	122.2
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	0.0	1.9
Investment Purchases	0.0	0.0
Investment Sales	0.0	(0.5)
Level 3 Transfers Into	0.0	0.0
Level 3 Transfers Out of	(89.5)	(34.1)
Ending Balance of Assets Measured with Unobservable Inputs	0.0	89.5
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	0.0	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
Level 3 Transfers Into		0.0
Level 3 Transfers Out of		(52.9)
Ending Balance of Assets Measured with Unobservable Inputs		0.0
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	265.3	274.1
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	0.3	(0.8)
Investment Purchases	8.0	0.0
Investment Sales	(4.8)	(3.7)
Level 3 Transfers Into	125.2	157.6
Level 3 Transfers Out of	(186.3)	(161.9)
Ending Balance of Assets Measured with Unobservable Inputs	207.7	265.3
Corporate Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	1,459.7	1,408.2
Investment Gain (Loss) included in Earnings	(0.6)	1.4
Investment Gain (Loss) included in OCI	2.8	6.9
Investment Purchases	118.9	115.9
Investment Sales	(149.2)	(111.2)
Level 3 Transfers Into	524.7	792.8
Level 3 Transfers Out of	(806.2)	(754.3)
Ending Balance of Assets Measured with Unobservable Inputs	1,150.1	1,459.7
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	23.2	23.8
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(0.4)	(0.6)
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	22.8	23.2
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	1,837.7	1,881.2
Investment Gain (Loss) included in Earnings	(0.6)	1.4
Investment Gain (Loss) included in OCI	2.7	7.4
Investment Purchases	126.9	115.9
Investment Sales	(154.0)	(115.4)
Level 3 Transfers Into	649.9	950.4
Level 3 Transfers Out of	(1,082.0)	(1,003.2)
Ending Balance of Assets Measured with Unobservable Inputs	1,380.6	1,837.7
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	1.2	1.4
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	0.0	0.1
Investment Purchases	0.0	0.0
Investment Sales	0.0	(0.3)
Level 3 Transfers Into	0.0	0.0
Level 3 Transfers Out of	(0.1)	0.0
Ending Balance of Assets Measured with Unobservable Inputs	$ 1.1	$ 1.2