Supplemental Oil and Gas Disclosures (Summary of Changes in Reserves) (Details) - ft³ ft³ in Billions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reserve Quantities [Line Items]
Proved reserves, beginning of year	14,775	5,253	6,215
Price revisions	154	1,691	(1,037)
Performance and production revisions	372	641	683
Total net revisions	526	2,332	(354)
Proved developed	177	1,258	257
Proved undeveloped	832	6,829	25
Total reserve additions	1,009	8,087	282
Production	(946)	(897)	(875)
Disposition of reserves in place	(3,443)		(15)
Proved reserves, end of year	11,921	14,775	5,253
Northeast Appalachia [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year	4,126	1,574	2,319
Price revisions	41	903	(794)
Performance and production revisions	107	154	318
Total net revisions	148	1,057	(476)
Proved developed	154	790	81
Proved undeveloped	397	1,100
Total reserve additions	551	1,890	81
Production	(459)	(395)	(350)
Proved reserves, end of year	4,366	4,126	1,574
Southwest Appalachia [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year	6,962	677	611
Price revisions	106	738	(127)
Performance and production revisions	272	125	199
Total net revisions	378	863	72
Proved developed	22	419	157
Proved undeveloped	435	5,186
Total reserve additions	457	5,605	157
Production	(243)	(183)	(148)
Disposition of reserves in place			(15)
Proved reserves, end of year	7,554	6,962	677
Fayetteville Shale [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year	3,679	2,997	3,281
Price revisions	6	49	(116)
Performance and production revisions	(6)	358	163
Total net revisions		407	47
Proved developed	1	48	19
Proved undeveloped		543	25
Total reserve additions	1	591	44
Production	(243)	(316)	(375)
Disposition of reserves in place	(3,437)
Proved reserves, end of year		3,679	2,997
Other Property [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year	8	5	4
Price revisions	1	1
Performance and production revisions	(1)	4	3
Total net revisions		5	3
Proved developed		1
Total reserve additions		1
Production	(1)	(3)	(2)
Disposition of reserves in place	(6)
Proved reserves, end of year	1	8	5