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, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	$ 1,399.5	$ 990.8	$ 1,141.4	$ 1,025.7
Earnings	0	(1.2)	(1.0)	(1.4)
Other Comprehensive Income or Loss	5.8	6.8	4.0	14.2
Purchases	18.6	44.4	18.6	61.1
Sales	(11.0)	(17.1)	(59.4)	(25.6)
Level 3 Transfers Into	213.0	163.9	312.6	233.7
Level 3 Transfers Out of	(800.7)	(399.1)	(591.0)	(519.2)
End of Year	825.2	788.5	825.2	788.5
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	66.4		68.1
Earnings	0		0
Other Comprehensive Income or Loss	0		0
Purchases	0		0
Sales	0		0
Level 3 Transfers Into	0		0
Level 3 Transfers Out of	(66.4)		(68.1)
End of Year	0		0
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	548.3	324.7	338.9	173.6
Earnings	0	0.7	0	0.7
Other Comprehensive Income or Loss	(0.8)	3.0	(3.6)	4.7
Purchases	0	0	0	0
Sales	(5.8)	(10.2)	(0.7)	(10.5)
Level 3 Transfers Into	61.1	89.1	97.5	180.7
Level 3 Transfers Out of	(440.0)	(189.4)	(269.3)	(131.3)
End of Year	162.8	217.9	162.8	217.9
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	21.3	32.2	31.7	0.7
Earnings	0	0	0	0
Other Comprehensive Income or Loss	0	(0.3)	0	(0.3)
Purchases	0	19.4	0	19.4
Sales	0	(0.1)	0	(0.1)
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	(20.7)	(31.5)	(31.1)	0
End of Year	0.6	19.7	0.6	19.7
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	733.2	633.8	665.5	829.7
Earnings	0	(1.9)	0	(2.1)
Other Comprehensive Income or Loss	7.8	3.3	7.3	8.9
Purchases	18.6	25.0	18.6	41.7
Sales	(5.2)	(6.8)	(44.4)	(15.0)
Level 3 Transfers Into	151.9	53.1	215.1	53.0
Level 3 Transfers Out of	(266.7)	(178.2)	(222.5)	(387.9)
End of Year	639.6	528.3	639.6	528.3
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	30.3	0.1	37.2	21.7
Earnings	0	0	(1.0)	0
Other Comprehensive Income or Loss	(1.2)	0.8	0.3	0.9
Purchases	0	0	0	0
Sales	0	0	(14.3)	0
Level 3 Transfers Into	0	21.7	0	0
Level 3 Transfers Out of	(6.9)	0	0	0
End of Year	22.2	22.6	22.2	22.6
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	4.5	1.5	11.2	1.5
Earnings	0	0	0	0
Other Comprehensive Income or Loss	0	0.1	0	0.1
Purchases	0	2.0	0	2.0
Sales	0	0	0	0
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	(0.1)	0	(6.8)	0
End of Year	4.4	3.6	4.4	3.6
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	(123.4)	(82.2)	(135.7)	(96.3)
Earnings	(3.3)	(4.8)	9.0	9.3
Other Comprehensive Income or Loss	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 Year	$ (126.7)	$ (87.0)	$ (126.7)	$ (87.0)