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, 2012		Dec. 31, 2011		Dec. 31, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 81.4		$ 72.7		$ (4.1)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	20.1		45.7		10.7
Interest Rate Swaps | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	77.9		50.3		28.1
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	40.0	[1]	34.8	[1]	29.5	[1]
Interest Rate Swaps | Net Realized Investments Gain (Loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0		0		0
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	4.1	[2]	3.5	[2]	7.3	[2]
Interest Rate Swaps | Interest and Debt Expenses
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0		0		0
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1.7)	[3]	(1.6)	[4]	(0.5)
Interest Rate Swaps | Other Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)					0
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)					(0.4)	[4]
Foreign Exchange Contracts | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0		0		0
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(5.3)	[1]	(1.1)	[1]	(1.9)	[1]
Foreign Exchange Contracts | Net Realized Investments Gain (Loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	3.5		22.4		(32.2)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(17.0)	[2]	10.1	[2]	(25.6)	[2]
Foreign Exchange Contracts | Interest and Debt Expenses
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)					0
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)					$ 2.3	[3]

[1] (1)Gain (loss) recognized in net investment income

[2] (2)Gain (loss) recognized in net realized investment gain (loss)

[3] (3)Loss recognized in interest and debt expense

[4] (2)Gain recognized in net realized investment gain (loss)