Supplemental Oil and Gas Disclosures (Summary of Changes in Reserves) (Details) - ft³ ft³ in Billions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reserve Quantities [Line Items]
Proved reserves, beginning of year	5,253	6,215	10,747
Price revisions	1,691	(1,037)	(5,718)
Performance and production revisions	641	683	1,635
Total net revisions	2,332	(354)	(4,083)
Proved developed	1,258	257	416
Proved undeveloped	6,829	25	176
Total reserve additions	8,087	282	592
Production	(897)	(875)	(976)
Acquisition of reserves in place			115
Disposition of reserves in place		(15)	(180)
Proved reserves, end of year	14,775	5,253	6,215
Northeast Appalachia [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year	1,574	2,319	3,191
Price revisions	903	(794)	(2,315)
Performance and production revisions	154	318	1,383
Total net revisions	1,057	(476)	(932)
Proved developed	790	81	202
Proved undeveloped	1,100		138
Total reserve additions	1,890	81	340
Production	(395)	(350)	(360)
Acquisition of reserves in place			80
Disposition of reserves in place
Proved reserves, end of year	4,126	1,574	2,319
Southwest Appalachia [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year	677	611	2,297
Price revisions	738	(127)	(1,875)
Performance and production revisions	125	199	209
Total net revisions	863	72	(1,666)
Proved developed	419	157	84
Proved undeveloped	5,186		4
Total reserve additions	5,605	157	88
Production	(183)	(148)	(143)
Acquisition of reserves in place			35
Disposition of reserves in place		(15)
Proved reserves, end of year	6,962	677	611
Fayetteville Shale [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year	2,997	3,281	5,069
Price revisions	49	(116)	(1,496)
Performance and production revisions	358	163	10
Total net revisions	407	47	(1,486)
Proved developed	48	19	129
Proved undeveloped	543	25	34
Total reserve additions	591	44	163
Production	(316)	(375)	(465)
Acquisition of reserves in place
Disposition of reserves in place
Proved reserves, end of year	3,679	2,997	3,281
Other Properties [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year	5	4	190
Price revisions	1		(32)
Performance and production revisions	4	3	33
Total net revisions	5	3	1
Proved developed	1		1
Total reserve additions	1		1
Production	(3)	(2)	(8)
Disposition of reserves in place			(180)
Proved reserves, end of year	8	5	4