Location and Fair Value Amounts of Derivative Instruments Reported in Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Sep. 11, 2012	Dec. 31, 2011
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 52.2		$ 47.7
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities		(3.4)	(11.9)
Current liabilities
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities		(1.1)	(11.1)
Long-term liabilities
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities		(2.3)	(0.8)
Current assets
Asset Derivatives
Fair Value of Derivative Instruments, Assets	34.0		25.2
Long-term assets
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 18.2		$ 22.5