Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Level 3) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
States, Municipalities, and Political Subdivisions
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	$ 156.1	$ 174.8	$ 175.1	$ 128.7
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	5.4	(10.0)	11.6	(8.3)
Purchases	0	0	0	0
Sales	(0.7)	(0.5)	(0.7)	(0.5)
Level 3 Transfers Into	0	0	0	32.1
Level 3 Transfers Out of	(29.3)	(12.3)	(54.5)	0
End of Period	131.5	152.0	131.5	152.0
Foreign Governments
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	80.4	82.6	78.5	82.1
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	0.8	(2.6)	2.7	(2.1)
Purchases	0	0	0	0
Sales	0	0	0	0
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	0	0	0	0
End of Period	81.2	80.0	81.2	80.0
Public Utilities
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	190.6	297.1	139.3	226.4
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	2.4	(1.1)	6.9	1.1
Purchases	0	0	0	0
Sales	(0.1)	0	(0.1)	(2.8)
Level 3 Transfers Into	58.5	0	105.6	36.0
Level 3 Transfers Out of	(72.0)	(220.5)	(72.3)	(185.2)
End of Period	179.4	75.5	179.4	75.5
Mortgage/Asset-backed Securities
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	0.4	0.5	0.5	0.5
Investment Gain (Loss) included in Earnings	(0.2)	0	(0.2)	0
Investment Gain (Loss) included in OCI	0.3	(1.0)	0.2	(0.9)
Purchases	0	0	0	15.3
Sales	(0.5)	0	(0.5)	0
Level 3 Transfers Into	0	15.4	0	0
Level 3 Transfers Out of	0	0	0	0
End of Period	0	14.9	0	14.9
All Other Corporate Bonds
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	1,593.0	1,097.7	1,923.3	1,525.8
Investment Gain (Loss) included in Earnings	0	0.8	0	1.5
Investment Gain (Loss) included in OCI	17.5	(57.7)	32.3	(77.7)
Purchases	48.0	38.6	89.6	95.0
Sales	(61.5)	(19.5)	(82.0)	(37.2)
Level 3 Transfers Into	259.4	790.0	302.6	829.6
Level 3 Transfers Out of	(696.7)	(528.7)	(1,106.1)	(1,015.8)
End of Period	1,159.7	1,321.2	1,159.7	1,321.2
Redeemable Preferred Stocks
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	24.4	24.8	23.8	24.8
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	0.4	(0.6)	1.0	(0.6)
Purchases	0	0	0	0
Sales	0	0	0	0
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	0	0	0	0
End of Period	24.8	24.2	24.8	24.2
Fixed Maturity Securities
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	2,044.9	1,677.5	2,340.5	1,988.3
Investment Gain (Loss) included in Earnings	(0.2)	0.8	(0.2)	1.5
Investment Gain (Loss) included in OCI	26.8	(73.0)	54.7	(88.5)
Purchases	48.0	38.6	89.6	110.3
Sales	(62.8)	(20.0)	(83.3)	(40.5)
Level 3 Transfers Into	317.9	805.4	408.2	897.7
Level 3 Transfers Out of	(798.0)	(761.5)	(1,232.9)	(1,201.0)
End of Period	1,576.6	1,667.8	1,576.6	1,667.8
Equity Securites
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	3.1	4.3	4.6	4.3
Investment Gain (Loss) included in Earnings	0	0	2.1	0
Investment Gain (Loss) included in OCI	0	0.1	(0.1)	0.1
Purchases	0	0	0	0
Sales	0	0	(3.5)	0
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	0	0	0	0
End of Period	3.1	4.4	3.1	4.4
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	(44.7)	(77.5)	(53.2)	(83.9)
Investment Gain (Loss) included in Earnings	12.4	12.4	20.9	18.8
Investment Gain (Loss) included in OCI	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	0	0	0	0
End of Period	$ (32.3)	$ (65.1)	$ (32.3)	$ (65.1)