Derivative Instruments - Outstanding Trading Contracts (Detail) (Forward Contracts)	Dec. 31, 2013 gal
Revenue
Derivative [Line Items]
Quantity in Gallons	1,892,000
Weighted Average Contract Prices	1.2786
Purchase
Derivative [Line Items]
Quantity in Gallons	1,991,000
Weighted Average Contract Prices	1.2444
Minimum | Revenue
Derivative [Line Items]
Estimated Market Prices	0.9900
Minimum | Purchase
Derivative [Line Items]
Estimated Market Prices	0.9411
Maximum | Revenue
Derivative [Line Items]
Estimated Market Prices	1.475
Maximum | Purchase
Derivative [Line Items]
Estimated Market Prices	1.4600