Supplemental Oil And Gas Disclosures (Summary Of Changes In Reserves) (Details) - ft³ ft³ in Billions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reserve Quantities [Line Items]
Proved reserves, beginning of year	6,215	10,747	6,976
Production	(875)	(976)	(768)
Disposition of reserves in place	(15)	(180)
Acquisition of reserves in place		115	2,303
Price revisions	(1,037)	(5,718)	54
Performance and production revisions	683	1,635	489
Total net revisions	(354)	(4,083)	543
Proved developed	257	416	531
Proved undeveloped	25	176	1,162
Total reserve additions	282	592	1,693
Proved reserves, end of year	5,253	6,215	10,747
Northeast Appalachia [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year	2,319	3,191	1,963
Production	(350)	(360)	(254)
Disposition of reserves in place
Acquisition of reserves in place		80	1
Price revisions	(794)	(2,315)	10
Performance and production revisions	318	1,383	636
Total net revisions	(476)	(932)	646
Proved developed	81	202	246
Proved undeveloped		138	589
Total reserve additions	81	340	835
Proved reserves, end of year	1,574	2,319	3,191
Southwest Appalachia [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year	611	2,297
Production	(148)	(143)	(3)
Disposition of reserves in place	(15)
Acquisition of reserves in place		35	2,300
Price revisions	(127)	(1,875)
Performance and production revisions	199	209
Total net revisions	72	(1,666)
Proved developed	157	84
Proved undeveloped		4
Total reserve additions	157	88
Proved reserves, end of year	677	611	2,297
Fayetteville Shale [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year	3,281	5,069	4,795
Production	(375)	(465)	(494)
Disposition of reserves in place
Acquisition of reserves in place
Price revisions	(116)	(1,496)	38
Performance and production revisions	163	10	(126)
Total net revisions	47	(1,486)	(88)
Proved developed	19	129	283
Proved undeveloped	25	34	573
Total reserve additions	44	163	856
Proved reserves, end of year	2,997	3,281	5,069
Other Properties [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year	4	190	218
Production	(2)	(8)	(17)
Disposition of reserves in place		(180)
Acquisition of reserves in place			2
Price revisions		(32)	6
Performance and production revisions	3	33	(21)
Total net revisions	3	1	(15)
Proved developed		1	2
Proved undeveloped
Total reserve additions		1	2
Proved reserves, end of year	5	4	190