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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	$ 136.7	$ 131.5	$ 140.1	$ 175.1
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	(2.4)	2.7	(4.7)	14.3
Investment Purchases	0.0	0.0	0.0	0.0
Investment Sales	0.0	(0.1)	(0.8)	(0.7)
Investment Level 3 Transfers Into	0.0	0.0	0.0	0.0
Investment Level 3 Transfers Out of	(7.7)	0.0	(8.0)	(54.6)
Assets Measured with Unobservable Inputs, Ending Balance	126.6	134.1	126.6	134.1
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	69.8	81.2	69.3	78.5
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	(1.5)	(0.2)	(1.0)	0.0
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	(44.5)	0.0	(42.0)
Assets Measured with Unobservable Inputs, Ending Balance	68.3	36.5	68.3	36.5
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	357.8	179.4	315.0	139.3
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	1.3	(0.5)	(2.9)	4.8
Investment Purchases	40.0	0.0	40.0	0.0
Investment Sales	0.0	0.0	(2.3)	(0.2)
Investment Level 3 Transfers Into	9.0	96.0	70.7	126.0
Investment Level 3 Transfers Out of	(88.6)	(35.2)	(101.0)	(30.2)
Assets Measured with Unobservable Inputs, Ending Balance	319.5	239.7	319.5	239.7
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance		0.0		0.5
Investment Gain (Loss) included in Earnings		0.0		(0.2)
Investment Gain (Loss) included in OCI		0.0		0.5
Investment Purchases		0.0		9.0
Investment Sales		0.0		(0.6)
Investment Level 3 Transfers Into		9.2		0.0
Investment Level 3 Transfers Out of		0.0		0.0
Assets Measured with Unobservable Inputs, Ending Balance		9.2		9.2
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1,321.9	1,159.7	1,425.3	1,923.3
Investment Gain (Loss) included in Earnings	0.0	1.0	2.1	1.0
Investment Gain (Loss) included in OCI	(13.9)	(2.7)	(35.8)	42.4
Investment Purchases	0.0	0.0	25.0	89.6
Investment Sales	(61.9)	(17.3)	(142.2)	(97.6)
Investment Level 3 Transfers Into	225.1	416.3	307.1	569.0
Investment Level 3 Transfers Out of	(456.1)	(198.3)	(566.4)	(1,169.0)
Assets Measured with Unobservable Inputs, Ending Balance	1,015.1	1,358.7	1,015.1	1,358.7
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	24.8	24.8	24.9	23.8
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)	1.1
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	24.8	24.9	24.8	24.9
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1,911.0	1,576.6	1,974.6	2,340.5
Investment Gain (Loss) included in Earnings	0.0	1.0	2.1	0.8
Investment Gain (Loss) included in OCI	(16.5)	(0.6)	(44.5)	63.1
Investment Purchases	40.0	0.0	65.0	98.6
Investment Sales	(61.9)	(17.4)	(145.3)	(99.1)
Investment Level 3 Transfers Into	234.1	521.5	377.8	695.0
Investment Level 3 Transfers Out of	(552.4)	(278.0)	(675.4)	(1,295.8)
Assets Measured with Unobservable Inputs, Ending Balance	1,554.3	1,803.1	1,554.3	1,803.1
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1.4	3.1	1.4	4.6
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	2.1
Investment Gain (Loss) included in OCI	0.1	0.0	0.1	(0.1)
Investment Purchases	0.0	0.0	0.0	0.0
Investment Sales	0.0	0.0	0.0	(3.5)
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	1.5	3.1	1.5	3.1
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance	(55.8)	(32.3)	(49.9)	(53.2)
Derivative Liability Gain (Loss) Included in Earnings	(27.3)	(2.3)	(33.2)	18.6
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	$ (83.1)	$ (34.6)	$ (83.1)	$ (34.6)