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	Jun. 30, 2021	Dec. 31, 2020
Derivatives, Fair Value [Line Items]
Derivative liability	$ (354,366)
Obligations
Derivatives, Fair Value [Line Items]
Derivative liability	(71,259)	$ (12,833)
Accounts receivable
Derivatives, Fair Value [Line Items]
Derivative asset	$ 0	13,050
Accounts payable
Derivatives, Fair Value [Line Items]
Derivative liability		$ (89,842)