Supplementary Information For Oil And Gas Producing Activities (Narrative) (Details) $ in Millions, ft³ in Billions	12 Months Ended
	Sep. 30, 2021 USD ($) ft³	Sep. 30, 2020 USD ($) ft³	Sep. 30, 2019 USD ($) ft³	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2018
Reserve Quantities [Line Items]
Amount spent for developing proved undeveloped reserves | $	$ 81.2	$ 219.9	$ 246.0
Proved Undeveloped Reserve (Volume)	636	551	1,018
Percentage of PUD reserves to the total proved reserves	16.50%	16.00%	33.00%
New PUD reserve additions	344	7
PUD Upward Revisions	9	20
PUD conversions to developed reserves	188	363
Proved Undeveloped Reserves, Removed	80	179
PUD Well Locations Removed	8	17		4	1
Decrease in Proved undeveloped (PUD) reserves		467
Increase in Proved undeveloped (PUD) reserves	85
Investment made to convert proved undeveloped reserves to developed reserves | $	$ 81.0	$ 220.0
Conversion Of Undeveloped Proved Reserves To Developed Proved Reserves After Revisions	198	393
Conversion of PUD to Developed as a Percentage of PUD Reserves Booked at End of Prior Year	34.00%	36.00%	39.00%	27.00%		51.00%
Upward Revisions to PUD Reserves Converted to Developed Reserves		30
Arbitrary discount rate	10.00%
Proved Undeveloped Reserves, Revisions		48
PUD Well Locations Added		5
Appalachian Region [Member]
Reserve Quantities [Line Items]
Well Locations Developed With Net PUD Reserves	18	35
Well Locations To Be Developed With Net PUD Reserves	53	99
Marcellus Shale Fields [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Volume)	220	287	383
PUD conversions to developed reserves	82	146
Proved Undeveloped Reserves, Removed	54	14
PUD Well Locations Removed	6	2
Utica Shale [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Volume)	411	265	632
PUD conversions to developed reserves	106	214
Proved Undeveloped Reserves, Removed	26	165
PUD Well Locations Removed	2	15
West Coast Region [Member]
Reserve Quantities [Line Items]
PUD conversions to developed reserves		3
Well Locations Developed With Net PUD Reserves		14
Total PUD Reserve Additions Estimated In The Next Fiscal Year [Member]
Reserve Quantities [Line Items]
Amount to be spent on developing proved undeveloped reserves | $	$ 161.0