Financial Instruments and Risk Management (Fair Value of Derivative Instruments Not Designated as Hedging Instruments) (Details) - Not Designated as Hedging Instrument - Commodity derivative contracts - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accounts Payable
Derivatives, Fair Value [Line Items]
Liabilities	$ 104,358
Accounts Receivable
Derivatives, Fair Value [Line Items]
Accounts receivable		$ 3,837