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, 2014	Mar. 31, 2013
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning of Period	$ 175.1	$ 128.7
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	6.5	2.7
Purchases	0	0
Sales	0	0
Level 3 Transfers Into	0	43.4
Level 3 Transfers Out of	(25.5)	0
End of Period	156.1	174.8
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning of Period	78.5	82.1
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	1.9	0.5
Purchases	0	0
Sales	0	0
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	0
End of Period	80.4	82.6
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning of Period	203.5	574.4
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	5.6	1.1
Purchases	6.6	0
Sales	0	(0.1)
Level 3 Transfers Into	124.5	252.3
Level 3 Transfers Out of	(30.2)	(425.2)
End of Period	310.0	402.5
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning of Period	0.5	0.5
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	0	0
Purchases	0	0
Sales	(0.1)	0
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	0
End of Period	0.4	0.5
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning of Period	1,859.1	1,177.8
Investment Gain (Loss) included in Earnings	0	0.8
Investment Gain (Loss) included in OCI	24.2	(8.8)
Purchases	73.0	30.3
Sales	(29.6)	(18.1)
Level 3 Transfers Into	362.9	382.4
Level 3 Transfers Out of	(816.0)	(572.1)
End of Period	1,473.6	992.3
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning of Period	23.8	24.8
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	0.6	0
Purchases	0	0
Sales	0	0
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	0
End of Period	24.4	24.8
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning of Period	2,340.5	1,988.3
Investment Gain (Loss) included in Earnings	0	0.8
Investment Gain (Loss) included in OCI	38.8	(4.5)
Purchases	79.6	30.3
Sales	(29.7)	(18.2)
Level 3 Transfers Into	487.4	678.1
Level 3 Transfers Out of	(871.7)	(997.3)
End of Period	2,044.9	1,677.5
Equity Securites
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning of Period	4.6	4.3
Investment Gain (Loss) included in Earnings	2.1	0
Investment Gain (Loss) included in OCI	(0.1)	0
Purchases	0	0
Sales	(3.5)	0
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	0
End of Period	3.1	4.3
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning of Period	(53.2)	(83.9)
Investment Gain (Loss) included in Earnings	8.5	6.4
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	$ (44.7)	$ (77.5)