Financial Instruments and Risk Management (Fair Value of Derivative Instruments Not Designated as Hedging Instruments) (Details) - Nondesignated [Member] - Accounts Payable [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Commodity Derivative Contracts [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	$ (39,093)	$ (48,864)
Foreign Exchange Derivative Contracts [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives		$ (73)