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, 2013	Mar. 31, 2012
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	$ 128.7	$ 68.1
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	2.7	(3.2)
Purchases	0	0
Sales	0	0
Level 3 Transfers Into	43.4	9.9
Level 3 Transfers Out of	0	(8.4)
End of Period	174.8	66.4
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	82.1
Investment Gain (Loss) included in Earnings	0
Investment Gain (Loss) included in OCI	0.5
Purchases	0
Sales	0
Level 3 Transfers Into	0
Level 3 Transfers Out of	0
End of Period	82.6
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	574.4	338.9
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	1.1	(0.1)
Purchases	0	0
Sales	(0.1)	(0.9)
Level 3 Transfers Into	252.3	416.9
Level 3 Transfers Out of	(425.2)	(206.5)
End of Period	402.5	548.3
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	0.5	31.7
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	0	2.4
Purchases	0	0
Sales	0	0
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	(12.8)
End of Period	0.5	21.3
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	1,177.8	665.5
Investment Gain (Loss) included in Earnings	0.8	0
Investment Gain (Loss) included in OCI	(8.8)	7.0
Purchases	30.3	12.0
Sales	(18.1)	(40.3)
Level 3 Transfers Into	382.4	223.0
Level 3 Transfers Out of	(572.1)	(134.0)
End of Period	992.3	733.2
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	24.8	37.2
Investment Gain (Loss) included in Earnings	0	(1.0)
Investment Gain (Loss) included in OCI	0	2.1
Purchases	0	0
Sales	0	(14.3)
Level 3 Transfers Into	0	6.3
Level 3 Transfers Out of	0	0
End of Period	24.8	30.3
Total Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	1,988.3	1,141.4
Investment Gain (Loss) included in Earnings	0.8	(1.0)
Investment Gain (Loss) included in OCI	(4.5)	8.2
Purchases	30.3	12.0
Sales	(18.2)	(55.5)
Level 3 Transfers Into	678.1	656.1
Level 3 Transfers Out of	(997.3)	(361.7)
End of Period	1,677.5	1,399.5
Equity Securites
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	4.3	11.2
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	0	0
Purchases	0	0
Sales	0	0
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	(6.7)
End of Period	4.3	4.5
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	(83.9)	(135.7)
Investment Gain (Loss) included in Earnings	6.4	12.3
Investment Gain (Loss) included in OCI	0	0
Purchases	0	0
Sales	0	0
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	0
End of Period	$ (77.5)	$ (123.4)