Financial Instruments and Risk Management (Fair Value of Derivative Instruments Not Designated as Hedging Instruments) (Details) - Not Designated as Hedging Instrument - Commodity derivative contracts - Accounts Payable - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Accounts receivable	$ 361,176
Accounts payable		$ (33,364)