Supplementary Information For Oil And Gas Producing Activities (Proved Developed And Undeveloped Oil And Gas Reserve Quantities) (Details) ft³ in Billions		12 Months Ended
		Sep. 30, 2016 MMcf MBbls ft³	Sep. 30, 2015 MMcf MBbls ft³	Sep. 30, 2014 MMcf MBbls ft³	Sep. 30, 2013 MMcf MBbls
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Volume) | ft³		543	827	512
Natural Gas (Mmcf) [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves		2,142,128	1,682,884	1,299,515
Extensions and Discoveries		185,347	633,360	446,821
Revisions of Previous Estimates		(248,161)	(34,441)	45,048
Production Volume		(143,547)	(139,563)	(142,307)
Purchases of Minerals in Place				33,986
Sales of Minerals in Place		(261,192)	(112)	(179)
Proved Developed and Undeveloped Reserves		1,674,575	2,142,128	1,682,884
Proved Developed Reserves (Volume)		1,132,616	1,316,844	1,177,808	866,917
Proved Undeveloped Reserve (Volume)		541,959	825,284	505,076	432,598
Oil Mbbl [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves | MBbls		33,722	38,477	41,598
Extensions and Discoveries | MBbls		530	533	1,539
Revisions of Previous Estimates | MBbls		(2,247)	(2,254)	(1,694)
Production Volume | MBbls		(2,923)	(3,034)	(3,036)
Purchases of Minerals in Place | MBbls				83
Sales of Minerals in Place | MBbls		(73)		(13)
Proved Developed and Undeveloped Reserves | MBbls		29,009	33,722	38,477
Proved Developed Reserves (Volume) | MBbls		28,771	33,370	37,255	38,365
Proved Undeveloped Reserve (Volume) | MBbls		238	352	1,222	3,233
West Coast Region [Member] | Natural Gas (Mmcf) [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves		49,346	58,822	60,777
Extensions and Discoveries		0	0	0
Revisions of Previous Estimates		(3,132)	(6,317)	1,358
Production Volume		(3,090)	(3,159)	(3,210)
Purchases of Minerals in Place				0
Sales of Minerals in Place		0	0	(103)
Proved Developed and Undeveloped Reserves		43,124	49,346	58,822
Proved Developed Reserves (Volume)		43,124	49,346	57,907	59,862
Proved Undeveloped Reserve (Volume)		0	0	915	915
West Coast Region [Member] | Oil Mbbl [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves | MBbls		33,502	38,224	41,315
Extensions and Discoveries | MBbls		530	533	1,521
Revisions of Previous Estimates | MBbls		(2,201)	(2,251)	(1,677)
Production Volume | MBbls		(2,895)	(3,004)	(3,005)
Purchases of Minerals in Place | MBbls				83
Sales of Minerals in Place | MBbls		0		(13)
Proved Developed and Undeveloped Reserves | MBbls		28,936	33,502	38,224
Proved Developed Reserves (Volume) | MBbls		28,698	33,150	37,002	38,082
Proved Undeveloped Reserve (Volume) | MBbls		238	352	1,222	3,233
Appalachian Region [Member] | Natural Gas (Mmcf) [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves		2,092,782	1,624,062	1,238,738
Extensions and Discoveries	[1]	185,347	633,360	446,821
Revisions of Previous Estimates		(245,029)	(28,124)	43,690
Production Volume	[2]	(140,457)	(136,404)	(139,097)
Purchases of Minerals in Place				33,986
Sales of Minerals in Place		(261,192)	(112)	(76)
Proved Developed and Undeveloped Reserves		1,631,451	2,092,782	1,624,062
Proved Developed Reserves (Volume)		1,089,492	1,267,498	1,119,901	807,055
Proved Undeveloped Reserve (Volume)		541,959	825,284	504,161	431,683
Appalachian Region [Member] | Oil Mbbl [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves | MBbls		220	253	283
Extensions and Discoveries | MBbls		0	0	18
Revisions of Previous Estimates | MBbls		(46)	(3)	(17)
Production Volume | MBbls		(28)	(30)	(31)
Purchases of Minerals in Place | MBbls				0
Sales of Minerals in Place | MBbls		(73)		0
Proved Developed and Undeveloped Reserves | MBbls		73	220	253
Proved Developed Reserves (Volume) | MBbls		73	220	253	283
Proved Undeveloped Reserve (Volume) | MBbls		0	0	0	0
Marcellus Shale Fields [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Volume) | ft³		542	825	504
Marcellus Shale Fields [Member] | Natural Gas (Mmcf) [Member]
Reserve Quantities [Line Items]
Extensions and Discoveries		179,000	598,000	442,000
Production Volume		(135,598)	(130,291)	(131,590)
Percentage exceeding total reserve of production in proved developed and undeveloped reserves		15.00%

[1] Extensions and discoveries include 442 Bcf (during 2014), 598 Bcf (during 2015) and 179 Bcf (during 2016), of Marcellus Shale gas in the Appalachian Region.

[2] Production includes 131,590 MMcf (during 2014), 130,291 MMcf (during 2015) and 135,598 MMcf (during 2016), from Marcellus Shale fields (which exceed 15% of total reserves). Oil Mbbl U. S. AppalachianRegion West CoastRegion TotalCompanyProved Developed and Undeveloped Reserves: September 30, 2013283 41,315 41,598Extensions and Discoveries18 1,521 1,539Revisions of Previous Estimates(17) (1,677) (1,694)Production(31) (3,005) (3,036)Purchases of Minerals in Place— 83 83Sales of Minerals in Place— (13) (13)September 30, 2014253 38,224 38,477Extensions and Discoveries— 533 533Revisions of Previous Estimates(3) (2,251) (2,254)Production(30) (3,004) (3,034)September 30, 2015220 33,502 33,722Extensions and Discoveries— 530 530Revisions of Previous Estimates(46) (2,201) (2,247)Production(28) (2,895) (2,923)Sales of Minerals in Place(73) — (73)September 30, 201673 28,936 29,009Proved Developed Reserves: September 30, 2013283 38,082 38,365September 30, 2014253 37,002 37,255September 30, 2015220 33,150 33,370September 30, 201673 28,698 28,771Proved Undeveloped Reserves: September 30, 2013— 3,233 3,233September 30, 2014— 1,222 1,222September 30, 2015— 352 352September 30, 2016— 238 238