Derivative Instruments - Outstanding Trading Contracts (Detail) (Forward Contracts [Member], USD $)	Dec. 31, 2012 gal
Revenue [Member]
Derivative [Line Items]
Quantity in Gallons	1,262,000
Weighted Average Contract Prices	$ 0.9214
Purchase [Member]
Derivative [Line Items]
Quantity in Gallons	2,648,000
Weighted Average Contract Prices	$ 0.9291
Minimum [Member] | Revenue [Member]
Derivative [Line Items]
Estimated Market Prices	$ 0.7550
Minimum [Member] | Purchase [Member]
Derivative [Line Items]
Estimated Market Prices	$ 0.7550
Maximum [Member] | Revenue [Member]
Derivative [Line Items]
Estimated Market Prices	$ 1.3650
Maximum [Member] | Purchase [Member]
Derivative [Line Items]
Estimated Market Prices	$ 1.3300