Supplemental Oil and Gas Disclosures (Unaudited) (Cost Incurred in Oil and Gas Property Acquisition, Exploration, and Development Activities Disclosure) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) $ / Mcfe	Dec. 31, 2019 USD ($) $ / Mcfe	Dec. 31, 2018 USD ($) $ / Mcfe
Natural Gas and Oil Properties [Line Items]
Unproved property acquisition costs	$ 124	$ 162	$ 164
Exploration costs	0	2	5
Development costs	784	936	1,014
Capitalized costs incurred	$ 908	$ 1,100	$ 1,183
Full cost pool amortization per Mcfe | $ / Mcfe	0.38	0.56	0.51
Montage Resources Corporation
Natural Gas and Oil Properties [Line Items]
Unproved property acquisition costs	$ 90