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	Mar. 31, 2021	Dec. 31, 2020
Derivatives, Fair Value [Line Items]
Derivative liability	$ (234,473)
Obligations
Derivatives, Fair Value [Line Items]
Derivative liability	(49,034)	$ (12,833)
Accounts receivable
Derivatives, Fair Value [Line Items]
Derivative asset		13,050
Accounts payable
Derivatives, Fair Value [Line Items]
Derivative liability	$ 0	$ (89,842)