Derivative Instruments - Fair Values of Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	$ 0	$ 0
Derivative Liability, Fair Value	(306.4)	(336.1)
Derivatives designated as hedging instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	0	0
Derivative Liability, Fair Value	0	0
Derivatives not designated as hedging instruments [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	0	0
Derivatives not designated as hedging instruments [Member] | Interest rate caps [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	0	0
Derivative Liability, Fair Value	0	0
Derivatives not designated as hedging instruments [Member] | Deferred credits and other [Member] | Interest rate swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value	$ (306.4)	$ (336.1)