Financial Instruments and Risk Management (Recognized Gains and Losses for Derivative Instruments Not Designated as Hedging Instruments) (Details) - Not Designated as Hedging Instrument - (Loss) gain on derivative instruments - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Commodity derivative contracts
Derivative Instruments, Gain (Loss) [Line Items]
(Loss) gain on derivative instruments	$ (43,235)	$ (5,290)	$ (483,865)	$ 319,502
Commodity collars
Derivative Instruments, Gain (Loss) [Line Items]
(Loss) gain on derivative instruments	$ (15,929)	$ 0	$ (15,929)	$ 0