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, 2013		Mar. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 1.9		$ 15.4
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	1.2		(3.8)
Interest Rate Swap [Member] | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	(2.2)		18.3
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	10.0	[1]	9.1	[1]
Interest Rate Swap [Member] | Net Realized Investments Gain Loss [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)			0.1	[2]
Interest Rate Swap [Member] | Interest and Debt Expenses [Member]
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)	[3]
Foreign Exchange Contract [Member] | 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)	(1.7)	[1]	(0.2)	[1]
Foreign Exchange Contract [Member] | Net Realized Investments Gain Loss [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	4.1		(2.9)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ (6.7)	[2]	$ (12.4)	[2]

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

[2] (2)Loss recognized in net realized investment gain

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