Supplemental Oil and Gas Disclosures (Narrative) (Details) $ in Millions, ft³ in Billions	12 Months Ended			144 Months Ended	180 Months Ended
	Dec. 31, 2018 USD ($) site ft³	Dec. 31, 2017 USD ($) site ft³	Dec. 31, 2016 USD ($) site ft³	Dec. 31, 2015 USD ($) ft³	Dec. 31, 2018 USD ($) site ft³
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization	$ 168	$ 138	$ 46	$ 1,403	$ 1,755
Impairments	171		2,321
Capitalized interest based on weighted average cost of borrowings	115	113	152
Capitalized internal costs related to acquisition, exploration and development	$ 90	$ 99	$ 87
Percentage of present worth of proved reserves evaluated in audit	99.00%	99.00%	99.00%
Proved developed and undeveloped reserves | ft³	11,921	14,775	5,253	6,215	11,921
Natural gas, oil and NGL reserves discount	10.00%
Undeveloped Properties Southwest Appalachia [Member]
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization	$ 1,500
Undeveloped Properties Northeast Appalachia [Member]
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization	23
Undeveloped Properties Outside the Appalachian Basin [Member]
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization	11
Wells In Progress [Member]
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization	77
Capitalized interest based on weighted average cost of borrowings	$ 184
Undeveloped Properties, Various Locations [Member]
Natural Gas and Oil Properties [Line Items]
Proved undeveloped reserves | ft³	190	1,375	77		190
Number of locations | site	30	330	15		30
Natural gas, oil and NGL reserves discount	10.00%	10.00%	10.00%
Present value of proved reserves, discounted basis	$ 24	$ 124	$ 11		$ 24
Proved reserves, committed development period	5 years