Disclosures About Fair Value Measurements (Narrative) (Details)	1 Months Ended		12 Months Ended
	Jul. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2020 USD ($) bblPerDay $ / bbl	Dec. 31, 2019 USD ($) $ / bbl	Dec. 31, 2018 USD ($)	May 31, 2020 USD ($)	May 31, 2019
Impairment of oil and gas properties			$ 0	$ 0	$ 77,000,000
Amortization of issuance discounts			$ 29,000,000	$ 1,000,000	$ 1,000,000
Purchase Contract, Amount Of Barrel To Be Purchased and Sold | bblPerDay			50,000
Long-term Purchase Commitment, Period			6 years
Marketing derivatives
Derivative, Fair Value Input, WASP Differential Deduction | $ / bbl			1.72	1.91
Senior notes
Debt Instrument, Unamortized Discount			$ 206,000,000
0.25% convertible senior notes due 2025 | Senior notes
Notes Payable, Fair Value Disclosure						$ 1,100,000,000
Debt Instrument, Unamortized Discount			206,000,000			$ 234,000,000
Amortization of issuance discounts			$ 28,000,000
Raton Basin
Impairment of oil and gas properties		$ 77,000,000
Contract deficiency	$ 111,000,000
Sand mine
Impairment of Long-Lived Assets to be Disposed of				$ 13,000,000
Pressure pumping assets
Impairment of Long-Lived Assets to be Disposed of				$ 16,000,000
Discount rate
Contingent Consideration, Cash Payment, Measurement Input							0.036
Contingent Consideration, Cash Receipt, Measurement Input							0.032
Oil and gas contracts			0.044