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		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	$ 144.2	$ 156.1	$ 140.1	$ 175.1
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	(6.7)	5.4	(2.6)	11.6
Investment Purchases	0.0	0.0	0.0	0.0
Investment Sales	(0.8)	(0.7)	(0.8)	(0.7)
Investment Level 3 Transfers Into	0.0	0.0	0.0	0.0
Investment Level 3 Transfers Out of	0.0	(29.3)	0.0	(54.5)
Assets Measured with Unobservable Inputs, Ending Balance	136.7	131.5	136.7	131.5
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	69.1	80.4	69.3	78.5
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	0.7	0.8	0.5	2.7
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	69.8	81.2	69.8	81.2
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	549.8	190.6	315.0	139.3
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	(4.7)	2.4	(3.6)	6.9
Investment Purchases	0.0	0.0	0.0	0.0
Investment Sales	(0.1)	(0.1)	(2.5)	(0.1)
Investment Level 3 Transfers Into	121.6	58.5	123.9	105.6
Investment Level 3 Transfers Out of	(308.8)	(72.0)	(75.0)	(72.3)
Assets Measured with Unobservable Inputs, Ending Balance	357.8	179.4	357.8	179.4
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance		0.4		0.5
Investment Gain (Loss) included in Earnings		(0.2)		(0.2)
Investment Gain (Loss) included in OCI		0.3		0.2
Investment Purchases		0.0		0.0
Investment Sales		(0.5)		(0.5)
Investment Level 3 Transfers Into		0.0		0.0
Investment Level 3 Transfers Out of		0.0		0.0
Assets Measured with Unobservable Inputs, Ending Balance		0.0		0.0
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1,635.8	1,593.0	1,425.3	1,923.3
Investment Gain (Loss) included in Earnings	2.4	0.0	2.1	0.0
Investment Gain (Loss) included in OCI	(45.3)	17.5	(36.0)	32.3
Investment Purchases	0.0	48.0	0.0	89.6
Investment Sales	(50.1)	(61.5)	(81.7)	(82.0)
Investment Level 3 Transfers Into	365.1	259.4	384.3	302.6
Investment Level 3 Transfers Out of	(586.0)	(696.7)	(372.1)	(1,106.1)
Assets Measured with Unobservable Inputs, Ending Balance	1,321.9	1,159.7	1,321.9	1,159.7
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	24.9	24.4	24.9	23.8
Investment Gain (Loss) included in Earnings	0.0	0.0	0.0	0.0
Investment Gain (Loss) included in OCI	(0.1)	0.4	(0.1)	1.0
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	0.0	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	24.8	24.8	24.8	24.8
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	2,423.8	2,044.9	1,974.6	2,340.5
Investment Gain (Loss) included in Earnings	2.4	(0.2)	2.1	(0.2)
Investment Gain (Loss) included in OCI	(56.1)	26.8	(41.8)	54.7
Investment Purchases	0.0	48.0	0.0	89.6
Investment Sales	(51.0)	(62.8)	(85.0)	(83.3)
Investment Level 3 Transfers Into	486.7	317.9	508.2	408.2
Investment Level 3 Transfers Out of	(894.8)	(798.0)	(447.1)	(1,232.9)
Assets Measured with Unobservable Inputs, Ending Balance	1,911.0	1,576.6	1,911.0	1,576.6
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	0.0	0.0	0.0	(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	1.4	3.1	1.4	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	(53.8)	(44.7)	(49.9)	(53.2)
Derivative Liability Gain (Loss) Included in Earnings	(2.0)	12.4	(5.9)	20.9
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	$ (55.8)	$ (32.3)	$ (55.8)	$ (32.3)