Location and Fair Value Amounts of Derivative Instruments Reported in Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Current liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	$ (8.3)
Long-term liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	(0.3)
Current Asset [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	14.4
Long-term assets [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	5.1
Commodity Contracts [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	19.5	48.9
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	(8.6)	(1.6)
Commodity Contracts [Member] | Current liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	(8.3)	(0.1)
Commodity Contracts [Member] | Long-term liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	(0.3)	(1.5)
Commodity Contracts [Member] | Current Asset [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	14.4	39.7
Commodity Contracts [Member] | Long-term assets [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 5.1	$ 9.2