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, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	$ 174.8	$ 66.4	$ 128.7	$ 68.1
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	(10.0)	0	(8.3)	0
Purchases	0	0	0	0
Sales	(0.5)	0	(0.5)	0
Level 3 Transfers Into	0	0	32.1	0
Level 3 Transfers Out of	(12.3)	(66.4)	0	(68.1)
End of Period	152.0	0	152.0	0
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	82.6		82.1
Investment Gain (Loss) included in Earnings	0		0
Investment Gain (Loss) included in OCI	(2.6)		(2.1)
Purchases	0		0
Sales	0		0
Level 3 Transfers Into	0		0
Level 3 Transfers Out of	0		0
End of Period	80.0		80.0
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	402.5	548.3	574.4	338.9
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	(14.4)	(0.8)	(16.9)	(3.6)
Purchases	8.6	0	38.1	0
Sales	0	(5.8)	(2.8)	(0.7)
Level 3 Transfers Into	230.3	61.1	265.7	97.5
Level 3 Transfers Out of	(272.8)	(440.0)	(504.3)	(269.3)
End of Period	354.2	162.8	354.2	162.8
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	0.5	21.3	0.5	31.7
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	(1.0)	0	(0.9)	0
Purchases	0	0	15.3	0
Sales	0	0	0	0
Level 3 Transfers Into	15.4	0	0	0
Level 3 Transfers Out of	0	(20.7)	0	(31.1)
End of Period	14.9	0.6	14.9	0.6
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	992.3	733.2	1,177.8	665.5
Investment Gain (Loss) included in Earnings	0.8	0	1.5	0
Investment Gain (Loss) included in OCI	(44.4)	7.8	(59.7)	7.3
Purchases	30.0	18.6	56.9	18.6
Sales	(19.5)	(5.2)	(37.2)	(44.4)
Level 3 Transfers Into	559.7	151.9	599.9	215.1
Level 3 Transfers Out of	(476.4)	(266.7)	(696.7)	(222.5)
End of Period	1,042.5	639.6	1,042.5	639.6
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	24.8	30.3	24.8	37.2
Investment Gain (Loss) included in Earnings	0	0	0	(1.0)
Investment Gain (Loss) included in OCI	(0.6)	(1.2)	(0.6)	0.3
Purchases	0	0	0	0
Sales	0	0	0	(14.3)
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	0	(6.9)	0	0
End of Period	24.2	22.2	24.2	22.2
Total Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	1,677.5	1,399.5	1,988.3	1,141.4
Investment Gain (Loss) included in Earnings	0.8	0	1.5	(1.0)
Investment Gain (Loss) included in OCI	(73.0)	5.8	(88.5)	4.0
Purchases	38.6	18.6	110.3	18.6
Sales	(20.0)	(11.0)	(40.5)	(59.4)
Level 3 Transfers Into	805.4	213.0	897.7	312.6
Level 3 Transfers Out of	(761.5)	(800.7)	(1,201.0)	(591.0)
End of Period	1,667.8	825.2	1,667.8	825.2
Equity Securites
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	4.3	4.5	4.3	11.2
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	0.1	0	0.1	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	(0.1)	0	(6.8)
End of Period	4.4	4.4	4.4	4.4
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	(77.5)	(123.4)	(83.9)	(135.7)
Investment Gain (Loss) included in Earnings	12.4	(3.3)	18.8	9.0
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	$ (65.1)	$ (126.7)	$ (65.1)	$ (126.7)