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, 2013	Dec. 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	(13.1)	(0.3)
Purchases	0	18.4
Sales	(1.0)	(0.7)
Level 3 Transfers Into	60.5	43.2
Level 3 Transfers Out of	0	0
End of Period	(175.1)	(128.7)
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	(82.1)	0
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	(3.6)	5.0
Purchases	0	15.4
Sales	0	0
Level 3 Transfers Into	0	61.7
Level 3 Transfers Out of	0	0
End of Period	(78.5)	(82.1)
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	(10.8)	22.3
Purchases	0	47.9
Sales	(3.1)	(4.2)
Level 3 Transfers Into	151.8	481.4
Level 3 Transfers Out of	(508.8)	(311.9)
End of Period	(203.5)	(574.4)
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	0.1	0
Purchases	0	0
Sales	(0.1)	(0.1)
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	(31.1)
End of Period	(0.5)	(0.5)
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	1.1	1.2
Investment Gain (Loss) included in OCI	(147.1)	40.8
Purchases	186.7	151.6
Sales	(122.0)	(67.5)
Level 3 Transfers Into	1,462.0	599.8
Level 3 Transfers Out of	(699.4)	(213.6)
End of Period	(1,859.1)	(1,177.8)
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	(1.0)	2.9
Purchases	0	0
Sales	0	(14.3)
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	0
End of Period	(23.8)	(24.8)
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	1.1	0.2
Investment Gain (Loss) included in OCI	(175.5)	70.7
Purchases	186.7	233.3
Sales	(126.2)	(86.8)
Level 3 Transfers Into	1,674.3	1,186.1
Level 3 Transfers Out of	(1,208.2)	(556.6)
End of Period	(2,340.5)	(1,988.3)
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	(0.1)
Investment Gain (Loss) included in OCI	0.3	0
Purchases	0	0
Sales	0	(0.1)
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	(6.7)
End of Period	(4.6)	(4.3)
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	30.7	51.8
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	$ (53.2)	$ (83.9)