Supplemental Oil and Gas Disclosures (Summary of Changes in Reserves) (Details) - Mcfe Mcfe in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]
Proved reserves, beginning of period, (bcfe)	11,921	14,775	5,253
Net revisions
Revisions of previous estimates due to price	(717)	154	1,691
Performance and production revisions	1,102	372	641
Total net revisions	385	526	2,332
Extensions, discoveries and other additions
Proved developed	191	177	1,258
Proved undeveloped	1,004	832	6,829
Total reserve additions	1,195	1,009	8,087
Production	(778)	(946)	(897)
Acquisition of reserves in place	0	0	0
Disposition of reserves in place	(2)	(3,443)	0
Proved reserves, end of period, (bcfe)	11,921	14,775	5,253	12,721
Proved and unproved reserves reclassified				109
Northeast Appalachia
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]
Proved reserves, beginning of period, (bcfe)	4,366	4,126	1,574
Net revisions
Revisions of previous estimates due to price	(57)	41	903
Performance and production revisions	127	107	154
Total net revisions	70	148	1,057
Extensions, discoveries and other additions
Proved developed	185	154	790
Proved undeveloped	677	397	1,100
Total reserve additions	862	551	1,890
Production	(459)	(459)	(395)
Acquisition of reserves in place	0	0	0
Disposition of reserves in place	(2)	0	0
Proved reserves, end of period, (bcfe)	4,366	4,126	1,574	4,837
Southwest Appalachia
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]
Proved reserves, beginning of period, (bcfe)	7,554	6,962	677
Net revisions
Revisions of previous estimates due to price	(660)	106	738
Performance and production revisions	975	272	125
Total net revisions	315	378	863
Extensions, discoveries and other additions
Proved developed	6	22	419
Proved undeveloped	327	435	5,186
Total reserve additions	333	457	5,605
Production	(319)	(243)	(183)
Acquisition of reserves in place	0	0	0
Disposition of reserves in place	0	0	0
Proved reserves, end of period, (bcfe)	7,554	6,962	677	7,883
Fayetteville Shale
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]
Proved reserves, beginning of period, (bcfe)	0	3,679	2,997
Net revisions
Revisions of previous estimates due to price	0	6	49
Performance and production revisions	0	(6)	358
Total net revisions	0	0	407
Extensions, discoveries and other additions
Proved developed	0	1	48
Proved undeveloped	0	0	543
Total reserve additions	0	1	591
Production	0	(243)	(316)
Acquisition of reserves in place	0	0	0
Disposition of reserves in place	0	(3,437)	0
Proved reserves, end of period, (bcfe)	0	3,679	2,997	0
Other
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]
Proved reserves, beginning of period, (bcfe)	1	8	5
Net revisions
Revisions of previous estimates due to price	0	1	1
Performance and production revisions	0	(1)	4
Total net revisions	0	0	5
Extensions, discoveries and other additions
Proved developed	0	0	1
Proved undeveloped	0	0	0
Total reserve additions	0	0	1
Production	0	(1)	(3)
Acquisition of reserves in place	0	0	0
Disposition of reserves in place	0	(6)	0
Proved reserves, end of period, (bcfe)	1	8	5	1