Supplemental Oil and Gas Disclosures (Unaudited) (Narrative) (Details) Mcfe in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) Mcfe location	Dec. 31, 2019 USD ($) Mcfe location	Dec. 31, 2018 USD ($) Mcfe location	Dec. 31, 2017 Mcfe
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization, cumulative	$ 1,472	$ 1,506
Capitalized interest	188
Capitalized interest based on weighted average cost of borrowings	88	109	$ 115
Capitalized internal costs related to acquisition, exploration and development	$ 56	$ 77	$ 90
Percentage of present worth of proved reserves evaluated in audit	97.00%	99.00%	99.00%
Proved reserves, end of period, (bcfe) | Mcfe	11,990	12,721	11,921	14,775
Proved undeveloped reverses (energy) | Mcfe	2,437	929	190
Number of locations | location	138	90	30
Present value of proved reserves, discounted basis	$ 207	$ 50	$ 24
Proved reserves, committed development period	5 years
Montage Resources Corporation
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization, cumulative	$ 88
Undeveloped Properties Southwest Appalachia
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization, cumulative	1,100
Undeveloped Properties Northeast Appalachia
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization, cumulative	6
Wells In Progress
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization, cumulative	61
Capitalized interest	$ 188
United States
Natural Gas and Oil Properties [Line Items]
Proved reserves, end of period, (bcfe) | Mcfe	11,990	12,721	11,921	14,775
Proved undeveloped reverses (energy) | Mcfe	3,787	6,300	6,364