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	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	$ 1,025.7	$ 869.4
Earnings	(2.2)	(6.5)
Other Comprehensive Income or Loss	38.7	66.3
Purchases	228.3	105.3
Sales	(84.7)	(70.1)
Level 3 Transfers Into	561.9	565.1
Level 3 Transfers Out of	(626.3)	(503.8)
End of Year	1,141.4	1,025.7
US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	0
Earnings	0
Other Comprehensive Income or Loss	2.5
Purchases	28.0
Sales	0
Level 3 Transfers Into	37.6
Level 3 Transfers Out of	0
End of Year	68.1
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	173.6	264.3
Earnings	0.2	(1.0)
Other Comprehensive Income or Loss	26.9	11.0
Purchases	34.8	0
Sales	(3.2)	(7.5)
Level 3 Transfers Into	259.9	120.5
Level 3 Transfers Out of	(153.3)	(213.7)
End of Year	338.9	173.6
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	0.7	4.7
Earnings	0	0
Other Comprehensive Income or Loss	(0.8)	0.3
Purchases	31.9	0
Sales	(0.1)	(4.3)
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	0
End of Year	31.7	0.7
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	829.7	580.0
Earnings	(2.4)	(5.5)
Other Comprehensive Income or Loss	8.9	53.7
Purchases	133.6	105.3
Sales	(81.4)	(58.3)
Level 3 Transfers Into	250.1	444.6
Level 3 Transfers Out of	(473.0)	(290.1)
End of Year	665.5	829.7
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	21.7	20.4
Earnings	0	0
Other Comprehensive Income or Loss	1.2	1.3
Purchases	0	0
Sales	0	0
Level 3 Transfers Into	14.3	0
Level 3 Transfers Out of	0	0
End of Year	37.2	21.7
Equity Securites
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	1.5	1.5
Earnings	(0.6)	0
Other Comprehensive Income or Loss	(1.7)	0.1
Purchases	3.0	0
Sales	0	(0.1)
Level 3 Transfers Into	9.0	0
Level 3 Transfers Out of	0	0
End of Year	11.2	1.5
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	(96.3)	(117.4)
Earnings	(39.4)	21.1
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	$ (135.7)	$ (96.3)