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, 2014	Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Level 3 Transfers Into	$ 0
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Assets Measured with Unobservable Inputs, Beginning Balance	175.1	128.7
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	21.0	(13.1)
Investment Purchases	0	0
Investment Sales	(1.4)	(1.0)
Level 3 Transfers Into	0	60.5
Level 3 Transfers Out of	(54.6)	0
Assets Measured with Unobservable Inputs, Ending Balance	140.1	175.1
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Assets Measured with Unobservable Inputs, Beginning Balance	78.5	82.1
Investment Gain (Loss) included in Earnings	1.1	0
Investment Gain (Loss) included in OCI	0.8	(3.6)
Investment Purchases	0	0
Investment Sales	(11.1)	0
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	0
Assets Measured with Unobservable Inputs, Ending Balance	69.3	78.5
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Assets Measured with Unobservable Inputs, Beginning Balance	139.3	226.4
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	6.9	(1.1)
Investment Purchases	0	0
Investment Sales	(0.8)	(3.1)
Level 3 Transfers Into	199.9	101.9
Level 3 Transfers Out of	(30.3)	(184.8)
Assets Measured with Unobservable Inputs, Ending Balance	315.0	139.3
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Assets Measured with Unobservable Inputs, Beginning Balance	0.5	0.5
Investment Gain (Loss) included in Earnings	(0.2)	0
Investment Gain (Loss) included in OCI	0.3	0.1
Investment Purchases	0	0
Investment Sales	(0.6)	(0.1)
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	0
Assets Measured with Unobservable Inputs, Ending Balance	0	0.5
Corporate Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Assets Measured with Unobservable Inputs, Beginning Balance	1,923.3	1,525.8
Investment Gain (Loss) included in Earnings	0.7	1.1
Investment Gain (Loss) included in OCI	44.8	(156.8)
Investment Purchases	91.1	186.7
Investment Sales	(147.7)	(122.0)
Level 3 Transfers Into	626.9	1,511.9
Level 3 Transfers Out of	(1,113.8)	(1,023.4)
Assets Measured with Unobservable Inputs, Ending Balance	1,425.3	1,923.3
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Assets Measured with Unobservable Inputs, Beginning Balance	23.8	24.8
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	1.1	(1.0)
Investment Purchases	0	0
Investment Sales	0	0
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	0
Assets Measured with Unobservable Inputs, Ending Balance	24.9	23.8
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Assets Measured with Unobservable Inputs, Beginning Balance	2,340.5	1,988.3
Investment Gain (Loss) included in Earnings	1.6	1.1
Investment Gain (Loss) included in OCI	74.9	(175.5)
Investment Purchases	91.1	186.7
Investment Sales	(161.6)	(126.2)
Level 3 Transfers Into	826.8	1,674.3
Level 3 Transfers Out of	(1,198.7)	(1,208.2)
Assets Measured with Unobservable Inputs, Ending Balance	1,974.6	2,340.5
Equity Securites
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Assets Measured with Unobservable Inputs, Beginning Balance	4.6	4.3
Investment Gain (Loss) included in Earnings	10.5	0
Investment Gain (Loss) included in OCI	(0.2)	0.3
Investment Purchases	0	0
Investment Sales	(13.5)	0
Level 3 Transfers Into	0	0
Level 3 Transfers Out of	0	0
Assets Measured with Unobservable Inputs, Ending Balance	1.4	4.6
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Derivatives Measured with Unobservable Inputs, Beginning Balance	(53.2)	(83.9)
Derivative Liability Gain (Loss) Included in Earnings	3.3	30.7
Derivative Liability Gain (Loss) Included in OCI	0	0
Derivative Purchases	0	0
Derivative Sales	0	0
Derivative Transfers Into Level 3	0	0
Derivative Transfers Out of Level 3	0	0
Derivatives Measured with Unobservable Inputs, Ending Balance	$ (49.9)	$ (53.2)