Derivative Instruments - Fair Values of Derivative Instruments (Detail) (Derivatives not designated as hedging instruments [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	$ 0		$ 0
Derivative Liability, Fair Value	(165.9)		(306.4)
Interest Rate Swap [Member] | Deferred Credits and Other [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	0		0
Derivative Liability, Fair Value	(165.9)		(306.4)
Interest Rate Cap [Member] | Deferred Credits and Other [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	0	[1]	0	[1]
Derivative Liability, Fair Value	$ 0		$ 0

[1] Amount rounds to zero.