Supplemental Oil and Gas Disclosures (Unaudited) (Summary of Changes in Reserves) (Details) - Mcfe Mcfe in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]
Proved reserves, beginning of period, (bcfe)	12,721	11,921	14,775
Net revisions
Price revisions	(4,370)	(717)	154
Performance and production revisions	1,424	1,102	372
Total net revisions	(2,946)	385	526
Extensions, discoveries and other additions
Proved developed	267	191	177
Proved undeveloped	474	1,004	832
Total reserve additions	741	1,195	1,009
Production	(880)	(778)	(946)
Acquisition of reserves in place	2,354	0	0
Disposition of reserves in place	0	(2)	(3,443)
Proved reserves, end of period, (bcfe)	12,721	11,921	14,775	11,990	12,721
Proved and unproved reserves reclassified					109
Northeast Appalachia
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]
Proved reserves, beginning of period, (bcfe)	4,837	4,366	4,126
Net revisions
Price revisions	(389)	(57)	41
Performance and production revisions	46	127	107
Total net revisions	(343)	70	148
Extensions, discoveries and other additions
Proved developed	198	185	154
Proved undeveloped	474	677	397
Total reserve additions	672	862	551
Production	(473)	(459)	(459)
Acquisition of reserves in place	223	0	0
Disposition of reserves in place	0	(2)	0
Proved reserves, end of period, (bcfe)	4,837	4,366	4,126	4,916	4,837
Southwest Appalachia
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]
Proved reserves, beginning of period, (bcfe)	7,883	7,554	6,962
Net revisions
Price revisions	(3,981)	(660)	106
Performance and production revisions	1,378	975	272
Total net revisions	(2,603)	315	378
Extensions, discoveries and other additions
Proved developed	69	6	22
Proved undeveloped	0	327	435
Total reserve additions	69	333	457
Production	(407)	(319)	(243)
Acquisition of reserves in place	2,131	0	0
Disposition of reserves in place	0	0	0
Proved reserves, end of period, (bcfe)	7,883	7,554	6,962	7,073	7,883
Fayetteville Shale
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]
Proved reserves, beginning of period, (bcfe)	0	0	3,679
Net revisions
Price revisions	0	0	6
Performance and production revisions	0	0	(6)
Total net revisions	0	0	0
Extensions, discoveries and other additions
Proved developed	0	0	1
Proved undeveloped	0	0	0
Total reserve additions	0	0	1
Production	0	0	(243)
Acquisition of reserves in place	0	0	0
Disposition of reserves in place	0	0	(3,437)
Proved reserves, end of period, (bcfe)	0	0	3,679	0	0
Other
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]
Proved reserves, beginning of period, (bcfe)	1	1	8
Net revisions
Price revisions	0	0	1
Performance and production revisions	0	0	(1)
Total net revisions	0	0	0
Extensions, discoveries and other additions
Proved developed	0	0	0
Proved undeveloped	0	0	0
Total reserve additions	0	0	0
Production	0	0	(1)
Acquisition of reserves in place	0	0	0
Disposition of reserves in place	0	0	(6)
Proved reserves, end of period, (bcfe)	1	1	8	1	1