Location of Gains and Losses on Derivative Financial Instruments Designated as Cash Flow Hedging Instruments (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 13.6	$ 29.3	$ 15.5	$ 44.7
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	4.3	21.1	5.5	17.3
Interest Rate Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	(4.9)	16.6	(7.1)	34.9
Interest Rate Swaps | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	11.1	9.6	21.1	18.7
Interest Rate Swaps | Net Realized Investments Gain Loss
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	0.8	2.0	0.8	2.1
Interest Rate Swaps | Interest and Debt Expenses
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(0.4)	(0.4)	(0.8)	(0.8)
Options Held
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	(0.1)	0	(0.1)	0
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	18.6	12.7	22.7	9.8
Foreign Exchange Contracts | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1.5)	(0.6)	(3.2)	(0.8)
Foreign Exchange Contracts | Net Realized Investments Gain Loss
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ (5.7)	$ 10.5	$ (12.4)	$ (1.9)