Financial Instruments and Risk Management (Fair Value of Derivative Instruments Not Designated as Hedging Instruments) (Details) - Nondesignated [Member] - Commodity Derivative Contracts [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	$ 3,837
Accounts Payable [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives		$ (39,093)