Derivative Instruments (Fair Values of Derivative Instruments in Consolidated Balance Sheets) (Details) - Not Designated as Hedging Instrument [Member] - Commodity Forward Contracts [Member] - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	$ 5,895	$ 5,553
Fair value of derivative liabilities	10,007	21,762
Accounts receivable, net [Member]
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	2,221	3,465
Other assets, net [Member]
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	3,674	2,088
Accrued liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	3,196	9,325
Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	$ 6,811	$ 12,437