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
	Mar. 31, 2012		Mar. 31, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 15.4		$ 6.7
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(3.8)		(9.6)
Interest Rate Swaps | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	18.3		14.6
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	9.1	[1]	8.4	[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
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	0.1	[2]	1.2	[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
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(0.4)	[3]	(0.4)
Foreign Exchange Contracts | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0		0
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(0.2)	[1]	(0.3)	[1]
Foreign Exchange Contracts | Net Realized Investments Gain (Loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	(2.9)		(7.9)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ (12.4)	[2]	$ (18.5)	[2]

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

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

[3] Loss recognized in interest and debt expense