Location and Fair Value Amounts of Derivative Instruments Reported in Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current liabilities
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	$ (5.4)
Long-term liabilities
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	(3.0)
Current Asset
Asset Derivatives
Fair Value of Derivative Instruments, Assets	18.7
Long-term assets
Asset Derivatives
Fair Value of Derivative Instruments, Assets	6.8
Commodity Contracts
Asset Derivatives
Fair Value of Derivative Instruments, Assets	25.5	48.9
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	(8.4)	(1.6)
Commodity Contracts | Current liabilities
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	(5.4)	(0.1)
Commodity Contracts | Long-term liabilities
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	(3.0)	(1.5)
Commodity Contracts | Current Asset
Asset Derivatives
Fair Value of Derivative Instruments, Assets	18.7	39.7
Commodity Contracts | Long-term assets
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 6.8	$ 9.2