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, 2020 MMcf MBbls Bcf ft³	Sep. 30, 2019 MMcf MBbls ft³	Sep. 30, 2018 MMcf MBbls ft³	Sep. 30, 2017 MMcf MBbls
Reserve Quantities [Line Items]
Purchases of Minerals in Place | Bcf		684,141
Proved Undeveloped Reserve (Volume) | ft³		551	1,018	757
Natural Gas (Mmcf) [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves		2,949,519	2,357,342	1,973,120
Extensions and Discoveries		7,246	686,549	521,694
Revisions of Previous Estimates		(88,419)	103,508	93,435
Production Volume		(227,402)	(197,880)	(162,906)
Purchases of Minerals in Place		684,141
Sales of Minerals in Place				(68,001)
Proved Developed and Undeveloped Reserves		3,325,085	2,949,519	2,357,342
Proved Developed Reserves (Volume)		2,773,823	1,934,795	1,606,532	1,363,102
Proved Undeveloped Reserve (Volume)		551,262	1,014,724	750,810	610,018
Oil Mbbl [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves | MBbls		24,873	27,663	30,207
Extensions and Discoveries | MBbls		288	787	2,301
Revisions of Previous Estimates | MBbls		(713)	(1,254)	2,477
Production Volume		(2,348)	(2,323)	(2,535)
Sales of Minerals in Place | MBbls				(4,787)
Proved Developed and Undeveloped Reserves | MBbls		22,100	24,873	27,663
Proved Developed Reserves (Volume) | MBbls		22,100	24,259	26,703	29,799
Proved Undeveloped Reserve (Volume) | MBbls		0	614	960	408
West Coast Region [Member] | Natural Gas (Mmcf) [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves		33,633	36,840	46,506
Extensions and Discoveries		0	0	0
Revisions of Previous Estimates		(2,772)	(1,233)	3,322
Production Volume		(1,889)	(1,974)	(2,407)
Purchases of Minerals in Place		0
Sales of Minerals in Place				(10,581)
Proved Developed and Undeveloped Reserves		28,972	33,633	36,840
Proved Developed Reserves (Volume)		28,972	33,633	36,840	46,506
Proved Undeveloped Reserve (Volume)		0	0	0	0
West Coast Region [Member] | Oil Mbbl [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves | MBbls		24,860	27,649	30,179
Extensions and Discoveries | MBbls		288	787	2,301
Revisions of Previous Estimates | MBbls		(715)	(1,256)	2,487
Production Volume		(2,345)	(2,320)	(2,531)
Sales of Minerals in Place | MBbls				(4,787)
Proved Developed and Undeveloped Reserves | MBbls		22,088	24,860	27,649
Proved Developed Reserves (Volume) | MBbls		22,088	24,246	26,689	29,771
Proved Undeveloped Reserve (Volume) | MBbls		0	614	960	408
Appalachian Region [Member] | Natural Gas (Mmcf) [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves		2,915,886	2,320,502	1,926,614
Extensions and Discoveries	[1]	7,246	686,549	521,694
Revisions of Previous Estimates		(85,647)	104,741	90,113
Production Volume	[2]	(225,513)	(195,906)	(160,499)
Purchases of Minerals in Place		684,141
Sales of Minerals in Place				(57,420)
Proved Developed and Undeveloped Reserves		3,296,113	2,915,886	2,320,502
Proved Developed Reserves (Volume)		2,744,851	1,901,162	1,569,692	1,316,596
Proved Undeveloped Reserve (Volume)		551,262	1,014,724	750,810	610,018
Appalachian Region [Member] | Oil Mbbl [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves | MBbls		13	14	28
Extensions and Discoveries | MBbls		0	0	0
Revisions of Previous Estimates | MBbls		2	2	(10)
Production Volume		(3)	(3)	(4)
Sales of Minerals in Place | MBbls				0
Proved Developed and Undeveloped Reserves | MBbls		12	13	14
Proved Developed Reserves (Volume) | MBbls		12	13	14	28
Proved Undeveloped Reserve (Volume) | MBbls		0	0	0	0
Marcellus Shale Fields [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Volume) | ft³		287	383	394
Marcellus Shale Fields [Member] | Natural Gas (Mmcf) [Member]
Reserve Quantities [Line Items]
Extensions and Discoveries		7,000	175,000	274,000
Production Volume		(169,453)	(163,015)	(150,196)
Percentage exceeding total reserve of production in proved developed and undeveloped reserves		15.00%
Utica Shale [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Volume) | ft³		265	632	357
Utica Shale [Member] | Natural Gas (Mmcf) [Member]
Reserve Quantities [Line Items]
Extensions and Discoveries		0	512,000	248,000
Production Volume		(55,392)	(32,095)	(9,409)
Percentage exceeding total reserve of production in proved developed and undeveloped reserves		15.00%

[1] Extensions and discoveries include 274 Bcf (during 2018), 175 Bcf (during 2019) and 7 Bcf (during 2020), of Marcellus Shale gas (which exceed 15% of total reserves) in the Appalachian region. Extensions and discoveries include 248 Bcf (during 2018), 512 Bcf (during 2019) and 0 Bcf (during 2020), of Utica Shale gas (which exceed 15% of total reserves) in the Appalachian region.

[2] Production includes 150,196 MMcf (during 2018), 163,015 MMcf (during 2019) and 169,453 MMcf (during 2020), from Marcellus Shale fields. Production includes 9,409 MMcf (during 2018), 32,095 MMcf (during 2019) and 55,392 MMcf (during 2020), from Utica Shale fields.