Supplemental Oil and Gas Disclosures (Narrative) (Details) Mcfe in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) Mcfe location	Dec. 31, 2018 USD ($) Mcfe location	Dec. 31, 2017 USD ($) Mcfe location	Dec. 31, 2016 Mcfe
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization, cumulative	$ 1,506	$ 1,755
Capitalized interest	179
Capitalized interest based on weighted average cost of borrowings	109	115	$ 113
Capitalized internal costs related to acquisition, exploration and development	$ 77	$ 90	$ 99
Percentage of present worth of proved reserves evaluated in audit	99.00%	99.00%	99.00%
Proved reserves, end of period, (bcfe) | Mcfe	12,721	11,921	14,775	5,253
Proved undeveloped reverses (energy) | Mcfe	929	190	1,375
Number of locations | location	90	30	330
Present value of proved reserves, discounted basis	$ 50	$ 24	$ 124
Natural gas, oil and NGL reserves discount	10.00%
Proved reserves, committed development period	5 years
Undeveloped Properties Southwest Appalachia
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization, cumulative	$ 1,200
Undeveloped Properties Northeast Appalachia
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization, cumulative	10
Wells In Progress
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization, cumulative	95
Capitalized interest	$ 179
United States
Natural Gas and Oil Properties [Line Items]
Proved reserves, end of period, (bcfe) | Mcfe	12,721	11,921	14,775	5,253
Proved undeveloped reverses (energy) | Mcfe	6,300	6,364	6,855