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, 2012	Dec. 31, 2011
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	$ 68.1	$ 0
Earnings	0	0
Other Comprehensive Income or Loss	(0.3)	2.5
Purchases	18.4	28.0
Sales	(0.7)	0
Level 3 Transfers Into	43.2	37.6
Level 3 Transfers Out of	0	0
End of Year	128.7	68.1
Foreign Government 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	5.0
Purchases	15.4
Sales	0
Level 3 Transfers Into	61.7
Level 3 Transfers Out of	0
End of Year	82.1
Public Utility, Bonds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	338.9	173.6
Earnings	0	0.2
Other Comprehensive Income or Loss	22.3	26.9
Purchases	47.9	34.8
Sales	(4.2)	(3.2)
Level 3 Transfers Into	481.4	259.9
Level 3 Transfers Out of	(311.9)	(153.3)
End of Year	574.4	338.9
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	0	(0.8)
Purchases	0	31.9
Sales	(0.1)	(0.1)
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	(31.1)	0
End of Year	0.5	31.7
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	665.5	829.7
Earnings	1.2	(2.4)
Other Comprehensive Income or Loss	40.8	8.9
Purchases	151.6	133.6
Sales	(67.5)	(81.4)
Level 3 Transfers Into	599.8	250.1
Level 3 Transfers Out of	(213.6)	(473.0)
End of Year	1,177.8	665.5
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	2.9	1.2
Purchases	0	0
Sales	(14.3)	0
Level 3 Transfers Into	0	14.3
Level 3 Transfers Out of	0	0
End of Year	24.8	37.2
Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	1,141.4	1,025.7
Earnings	0.2	(2.2)
Other Comprehensive Income or Loss	70.7	38.7
Purchases	233.3	228.3
Sales	(86.8)	(84.7)
Level 3 Transfers Into	1,186.1	561.9
Level 3 Transfers Out of	(556.6)	(626.3)
End of Year	1,988.3	1,141.4
Equity Securites
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Year	11.2	1.5
Earnings	(0.1)	(0.6)
Other Comprehensive Income or Loss	0	(1.7)
Purchases	0	3.0
Sales	(0.1)	0
Level 3 Transfers Into	0	9.0
Level 3 Transfers Out of	(6.7)	0
End of Year	4.3	11.2
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	51.8	(39.4)
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	$ (83.9)	$ (135.7)