Location of Gains and Losses on Derivative Financial Instruments Designated as Cash Flow Hedging Instruments (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 15.3	$ 81.4	$ 72.7
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	23.0	20.1	45.7
Interest Rate Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	(7.2)	77.9	50.3
Interest Rate Swaps | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	43.1	40.0	34.8
Interest Rate Swaps | Net Realized Investments Gain (Loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	1.3	4.1	3.5
Interest Rate Swaps | Interest and Debt Expenses
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1.7)	(1.7)	(1.6)
Options Held
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	(0.1)	0	0
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	22.6	3.5	22.4
Foreign Exchange Contracts | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(5.9)	(5.3)	(1.1)
Foreign Exchange Contracts | Net Realized Investments Gain (Loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ (13.8)	$ (17.0)	$ 10.1