Derivative Instruments - Outstanding Trading Contracts (Detail) - Forward Contracts gal in Thousands	Dec. 31, 2014 $ / gal gal
Revenue
Derivative [Line Items]
Quantity in Gallons | gal	4,200
Weighted Average Contract Prices	0.6714
Purchase
Derivative [Line Items]
Quantity in Gallons | gal	4,201
Weighted Average Contract Prices	0.6416
Minimum | Revenue
Derivative [Line Items]
Estimated Market Prices	0.5400
Minimum | Purchase
Derivative [Line Items]
Estimated Market Prices	0.4700
Maximum | Revenue
Derivative [Line Items]
Estimated Market Prices	0.7900
Maximum | Purchase
Derivative [Line Items]
Estimated Market Prices	1.3176