Supplemental Oil and Gas Disclosures (Narrative) (Details) ft³ in Thousands, $ in Millions	3 Months Ended				12 Months Ended			132 Months Ended	168 Months Ended
	Dec. 31, 2016 USD ($) site MMBbls ft³	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) $ / bbl $ / MMBTU site MMBbls ft³	Dec. 31, 2016 USD ($) $ / bbl $ / MMBTU site MMBbls ft³	Dec. 31, 2015 USD ($) $ / bbl $ / MMBTU site MMBbls ft³	Dec. 31, 2014 USD ($) MMBbls ft³	Dec. 31, 2017 USD ($) site MMBbls ft³	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)
Natural Gas and Oil Properties [Line Items]
Natural gas, oil and NGL reserves discount					10.00%	10.00%	10.00%
Period of time needed to calculate ceiling value of reserves					12 months	12 months	12 months
Cash flow hedges impact on ceiling value, net of tax | $										$ 40	$ 60
Net unevaluated costs excluded from amortization | $					$ 214	$ 66	$ 210	$ 1,327	$ 1,817
Impairment of natural gas and oil properties | $	$ 0	$ 817	$ 470	$ 1,034	$ 0	$ 2,321	$ 6,950
Impairment of natural gas and oil properties, net of tax | $		$ 506	$ 297	$ 641						$ 1,746	$ 944
Percentage of present worth of proved reserves evaluated in audit					99.00%	99.00%	100.00%
Capitalized interest based on weighted average cost of borrowings | $					$ 113	$ 152	$ 204
Capitalized internal costs related to acquisition, exploration and development | $					$ 99	$ 87	$ 175
Proved developed and undeveloped reserves	5,253,000,000				14,775,000,000	5,253,000,000	6,215,000,000	10,747,000,000	14,775,000,000
Production					897,000,000	875,000,000	976,000,000
Extensions, discoveries and other additions					8,087,000,000	282,000,000	592,000,000
Net revisions					2,332,000,000	(354,000,000)	(4,083,000,000)
Proved undeveloped reserves	77,000,000				6,855,000,000	77,000,000	443,000,000		6,855,000,000
Undeveloped Properties New Ventures - Excluding Canada [Member]
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization | $					$ 16
Wells In Progress [Member]
Natural Gas and Oil Properties [Line Items]
Net unevaluated costs excluded from amortization | $					88
Capitalized interest based on weighted average cost of borrowings | $					$ 159
Southwest Appalachia [Member]
Natural Gas and Oil Properties [Line Items]
Proved developed and undeveloped reserves	677,000,000				6,962,000,000	677,000,000	611,000,000	2,297,000,000	6,962,000,000
Production					183,000,000	148,000,000	143,000,000
Extensions, discoveries and other additions					5,605,000,000	157,000,000	88,000,000
Net revisions					863,000,000	72,000,000	(1,666,000,000)
Other Properties [Member]
Natural Gas and Oil Properties [Line Items]
Proved developed and undeveloped reserves	5,000,000				8,000,000	5,000,000	4,000,000	190,000,000	8,000,000
Production					3,000,000	2,000,000	8,000,000
Extensions, discoveries and other additions					1,000,000		1,000,000
Net revisions					5,000,000	3,000,000	1,000,000
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 | $					$ 90
Undeveloped Properties, Various Locations [Member]
Natural Gas and Oil Properties [Line Items]
Natural gas, oil and NGL reserves discount					10.00%	10.00%	10.00%
Proved undeveloped reserves	77,000,000				1,375,000,000	77,000,000	217,000,000		1,375,000,000
Number of locations | site	15				330	15	75		330
Present value of proved reserves, discounted basis | $	$ (11)				$ (124)	$ (11)	$ (34)		$ (124)
Proved reserves, committed development period					5 years
Natural Gas [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price | $ / MMBTU					2.98	2.48	2.59
Proved developed and undeveloped reserves	4,866,000,000				11,126,000,000	4,866,000,000	5,917,000,000	9,809,000,000	11,126,000,000
Production					797,000,000	788,000,000	899,000,000
Extensions, discoveries and other additions					5,159,000,000	198,000,000	546,000,000
Net revisions					1,898,000,000	(446,000,000)	(3,458,000,000)
Proved undeveloped reserves	77,000,000				4,147,000,000	77,000,000	443,000,000		4,147,000,000
Oil [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price | $ / bbl					47.79	39.25	46.79
Proved developed and undeveloped reserves | MMBbls	10,523				65,636	10,523	8,753	37,615	65,636
Production | MMBbls					2,327	2,192	2,265
Extensions, discoveries and other additions | MMBbls					55,772	2,417	1,367
Net revisions | MMBbls					1,668	1,564	(28,394)
Proved undeveloped reserves | MMBbls					51,123				51,123
NGL [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price | $ / bbl					14.41	6.74	6.82
Impairment of natural gas and oil properties, net of tax | $							$ 1,586
Proved developed and undeveloped reserves | MMBbls	53,931				542,455	53,931	40,947	118,699	542,455
Production | MMBbls					14,245	12,372	10,702
Extensions, discoveries and other additions | MMBbls					432,220	11,576	6,274
Net revisions | MMBbls					70,549	13,794	(75,664)
Proved undeveloped reserves | MMBbls					400,242				400,242
Natural Gas and Oil [Member]
Natural Gas and Oil Properties [Line Items]
Proved developed and undeveloped reserves	5,253,000,000				14,775,000,000	5,253,000,000			14,775,000,000
Synthetic Gas, Synthetic Oil or Nonrenewable Natural Resources [Member]
Natural Gas and Oil Properties [Line Items]
Proved developed and undeveloped reserves					0				0