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		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	$ 825.2	$ 788.5	$ 1,141.4	$ 1,025.7
Earnings	0	0.1	(1.0)	(2.0)
Other Comprehensive Income or Loss	31.9	9.6	45.4	21.4
Purchases	0	30.0	40.8	91.1
Sales	(7.0)	(29.5)	(67.4)	(48.3)
Level 3 Transfers Into	887.5	339.8	893.8	395.9
Level 3 Transfers Out of	(179.3)	(234.6)	(494.7)	(579.9)
End of Year	1,558.3	903.9	1,558.3	903.9
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	0		68.1
Earnings	0		0
Other Comprehensive Income or Loss	2.0		(0.9)
Purchases	0		0
Sales	0		(0.3)
Level 3 Transfers Into	108.1		43.2
Level 3 Transfers Out of	0		0
End of Year	110.1		110.1
Foreign Government Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	0		0
Earnings	0		0
Other Comprehensive Income or Loss	1.3		5.2
Purchases	0		0
Sales	0		0
Level 3 Transfers Into	44.8		40.9
Level 3 Transfers Out of	0		0
End of Year	46.1		46.1
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	162.8	217.9	338.9	173.6
Earnings	0	0.1	0	0.1
Other Comprehensive Income or Loss	8.4	9.3	5.4	17.1
Purchases	0	30.0	3.6	30.0
Sales	(1.7)	(11.9)	(3.7)	(15.7)
Level 3 Transfers Into	340.5	220.2	361.7	299.7
Level 3 Transfers Out of	(10.9)	(69.8)	(206.8)	(109.0)
End of Year	499.1	395.8	499.1	395.8
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	0.6	19.7	31.7	0.7
Earnings	0	0	0	0
Other Comprehensive Income or Loss	0.4	(0.1)	0.9	(0.4)
Purchases	0	0	7.0	19.4
Sales	0	0	(0.1)	(0.1)
Level 3 Transfers Into	7.4	0	0	0
Level 3 Transfers Out of	0	0	(31.1)	0
End of Year	8.4	19.6	8.4	19.6
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	639.6	528.3	665.5	829.7
Earnings	0	0	0	(2.1)
Other Comprehensive Income or Loss	19.8	1.3	34.0	4.7
Purchases	0	0	30.2	41.7
Sales	(5.3)	(17.6)	(49.0)	(32.5)
Level 3 Transfers Into	386.7	119.6	448.0	96.2
Level 3 Transfers Out of	(146.2)	(164.8)	(234.1)	(470.9)
End of Year	894.6	466.8	894.6	466.8
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	22.2	22.6	37.2	21.7
Earnings	0	0	(1.0)	0
Other Comprehensive Income or Loss	0	(0.9)	0.8	0
Purchases	0	0	0	0
Sales	0	0	(14.3)	0
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	(22.2)	0	(22.7)	0
End of Year	0	21.7	0	21.7
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	4.4	3.6	11.2	1.5
Earnings	0	(0.4)	0	(0.4)
Other Comprehensive Income or Loss	0	(2.4)	0	(1.7)
Purchases	0	0	0	2.0
Sales	0	0	(0.1)	0
Level 3 Transfers Into	0	9.6	0	9.0
Level 3 Transfers Out of	0	0	(6.7)	0
End of Year	4.4	10.4	4.4	10.4
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	(126.7)	(87.0)	(135.7)	(96.3)
Earnings	19.7	(33.7)	28.7	(24.4)
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	$ (107.0)	$ (120.7)	$ (107.0)	$ (120.7)