Derivative Instruments - Outstanding Trading Contracts (Details) - Forward Contracts [Member] gal in Thousands	Mar. 31, 2016 $ / gal gal
Sales [Member]
Derivative [Line Items]
Quantity In Gallons | gal	630
Weighted Average Contract Prices	0.4425
Sales [Member] | Minimum [Member]
Derivative [Line Items]
Estimated Market Prices	0.4425
Sales [Member] | Maximum [Member]
Derivative [Line Items]
Estimated Market Prices	0.4425
Purchase [Member]
Derivative [Line Items]
Quantity In Gallons | gal	631
Weighted Average Contract Prices	0.4422
Purchase [Member] | Minimum [Member]
Derivative [Line Items]
Estimated Market Prices	0.4413
Purchase [Member] | Maximum [Member]
Derivative [Line Items]
Estimated Market Prices	0.4413