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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
States, Municipalities, and Political Subdivisions
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	$ 131.5	$ 152.0	$ 175.1	$ 128.7
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	2.7	(0.1)	14.3	(8.4)
Purchases	0	0	0	0
Sales	(0.1)	0	(0.7)	(0.5)
Level 3 Transfers Into	0	0	0	32.1
Level 3 Transfers Out of	0	0	(54.6)	0
End of Period	134.1	151.9	134.1	151.9
Foreign Governments
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	81.2	80.0	78.5	82.1
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	(0.2)	(1.4)	0	(3.5)
Purchases	0	0	0	0
Sales	0	0	0	0
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	(44.5)	0	(42.0)	0
End of Period	36.5	78.6	36.5	78.6
Public Utilities
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	179.4	75.5	139.3	226.4
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	(0.5)	2.3	4.8	1.0
Purchases	0	0	0	0
Sales	0	0	(0.2)	(3.6)
Level 3 Transfers Into	96.0	93.8	126.0	114.9
Level 3 Transfers Out of	(35.2)	(18.1)	(30.2)	(185.2)
End of Period	239.7	153.5	239.7	153.5
Mortgage/Asset-backed Securities
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	0	14.9	0.5	0.5
Investment Gain (Loss) included in Earnings	0	(1.3)	(0.2)	0
Investment Gain (Loss) included in OCI	0	0.9	0.5	0.1
Purchases	0	0	9.0	0
Sales	0	(14.0)	(0.6)	(0.1)
Level 3 Transfers Into	9.2	0	0	0
Level 3 Transfers Out of	0	0	0	0
End of Period	9.2	0.5	9.2	0.5
All Other Corporate Bonds
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	1,159.7	1,321.2	1,923.3	1,525.8
Investment Gain (Loss) included in Earnings	1.0	(1.0)	1.0	1.1
Investment Gain (Loss) included in OCI	(2.7)	(10.0)	42.4	(90.9)
Purchases	0	7.0	89.6	30.1
Sales	(17.3)	(23.0)	(97.6)	(86.9)
Level 3 Transfers Into	416.3	591.0	569.0	937.2
Level 3 Transfers Out of	(198.3)	(534.4)	(1,169.0)	(965.6)
End of Period	1,358.7	1,350.8	1,358.7	1,350.8
Redeemable Preferred Stocks
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	24.8	24.2	23.8	24.8
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	0.1	(0.1)	1.1	(0.7)
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 Period	24.9	24.1	24.9	24.1
Fixed Maturity Securities
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	1,576.6	1,667.8	2,340.5	1,988.3
Investment Gain (Loss) included in Earnings	1.0	(2.3)	0.8	1.1
Investment Gain (Loss) included in OCI	(0.6)	(8.4)	63.1	(102.4)
Purchases	0	7.0	98.6	30.1
Sales	(17.4)	(37.0)	(99.1)	(91.1)
Level 3 Transfers Into	521.5	684.8	695.0	1,084.2
Level 3 Transfers Out of	(278.0)	(552.5)	(1,295.8)	(1,150.8)
End of Period	1,803.1	1,759.4	1,803.1	1,759.4
Equity Securites
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	3.1	4.4	4.6	4.3
Investment Gain (Loss) included in Earnings	0	0	2.1	0
Investment Gain (Loss) included in OCI	0	0	(0.1)	0.1
Purchases	0	0	0	0
Sales	0	0	(3.5)	0
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	0	0	0	0
End of Period	3.1	4.4	3.1	4.4
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value of Financial Instruments Measured on Recurring Basis with Unobservable Inputs Reconciliation
Beginning of Period	(32.3)	(65.1)	(53.2)	(83.9)
Investment Gain (Loss) included in Earnings	(2.3)	(0.3)	18.6	18.5
Investment Gain (Loss) included in OCI	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 Period	$ (34.6)	$ (65.4)	$ (34.6)	$ (65.4)