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
	Mar. 31, 2012	Mar. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	$ 1,141.4	$ 1,025.7
Earnings	(1.0)	(0.2)
Other Comprehensive Income or Loss	8.2	(1.8)
Purchases	12.0	23.7
Sales	(55.5)	(5.8)
Level 3 Transfers Into	656.1	438.1
Level 3 Transfers Out of	(361.7)	(488.9)
End of Year	1,399.5	990.8
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	68.1
Earnings	0
Other Comprehensive Income or Loss	(3.2)
Purchases	0
Sales	0
Level 3 Transfers Into	9.9
Level 3 Transfers Out of	(8.4)
End of Year	66.4
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	338.9	173.6
Earnings	0	0
Other Comprehensive Income or Loss	(0.1)	(0.7)
Purchases	0	7.0
Sales	(0.9)	(0.3)
Level 3 Transfers Into	416.9	249.7
Level 3 Transfers Out of	(206.5)	(104.6)
End of Year	548.3	324.7
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	31.7	0.7
Earnings	0	0
Other Comprehensive Income or Loss	2.4	(1.6)
Purchases	0	0
Sales	0	(2.2)
Level 3 Transfers Into	0	35.3
Level 3 Transfers Out of	(12.8)	0
End of Year	21.3	32.2
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	665.5	829.7
Earnings	0	(0.2)
Other Comprehensive Income or Loss	7.0	0.5
Purchases	12.0	16.7
Sales	(40.3)	(3.3)
Level 3 Transfers Into	223.0	153.1
Level 3 Transfers Out of	(134.0)	(362.7)
End of Year	733.2	633.8
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	37.2	21.7
Earnings	(1.0)	0
Other Comprehensive Income or Loss	2.1	0
Purchases	0	0
Sales	(14.3)	0
Level 3 Transfers Into	6.3	0
Level 3 Transfers Out of	0	(21.6)
End of Year	30.3	0.1
Equity Securites
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	11.2	1.5
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	(6.7)	0
End of Year	4.5	1.5
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	(135.7)	(96.3)
Earnings	12.3	14.1
Other Comprehensive Income or Loss	0	0
Purchases	0	0
Sales	0	0
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	0
End of Year	$ (123.4)	$ (82.2)