Supplementary Information For Oil And Gas Producing Activities (Narrative) (Details) $ in Millions, ft³ in Billions	12 Months Ended
	Sep. 30, 2020 USD ($) ft³	Sep. 30, 2019 USD ($) ft³	Sep. 30, 2018 USD ($) ft³	Sep. 30, 2017	Sep. 30, 2016
Reserve Quantities [Line Items]
Amount spent for developing proved undeveloped reserves | $	$ 219.9	$ 246.0	$ 182.3
Proved Undeveloped Reserve (Volume)	551	1,018	757
Percentage of PUD reserves to the total proved reserves	16.00%	33.00%	30.00%
New PUD reserve additions	7	575
PUD Upward Revisions	20	38
PUD conversions to developed reserves	363	297
Proved Undeveloped Reserves, Removed	179	55
PUD Well Locations Removed	17	6		4	1
Decrease in Proved undeveloped (PUD) reserves	467
Increase in Proved undeveloped (PUD) reserves		261
Investment made to convert proved undeveloped reserves to developed reserves | $	$ 220.0	$ 246.0
Conversion Of Undeveloped Proved Reserves To Developed Proved Reserves After Revisions	393	380
Conversion of PUD to Developed as a Percentage of PUD Reserves Booked at End of Prior Year	36.00%	39.00%	51.00%	27.00%	25.00%
Well Locations Developed With Net PUD Reserves		56
Percent of Well Locations Developed With Net PUD Reserves		50.00%
Upward Revisions to PUD Reserves Converted to Developed Reserves	30	83
Arbitrary discount rate	10.00%
Proved Undeveloped Reserves, Revisions	48
PUD Well Locations Added	5
West Coast Region [Member]
Reserve Quantities [Line Items]
New PUD reserve additions		2
PUD conversions to developed reserves	3	5
Well Locations Developed With Net PUD Reserves	14
Appalachian Region [Member]
Reserve Quantities [Line Items]
Well Locations Developed With Net PUD Reserves	35
Well Locations To Be Developed With Net PUD Reserves	99
Marcellus Shale Fields [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Volume)	287	383	394
New PUD reserve additions		175
PUD conversions to developed reserves	146	186
Proved Undeveloped Reserves, Removed	14	13
PUD Well Locations Removed	2	2
Utica Shale [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Volume)	265	632	357
New PUD reserve additions		398
PUD conversions to developed reserves	214	106
Proved Undeveloped Reserves, Removed	165	42
PUD Well Locations Removed	15	4
Total PUD Reserve Additions Estimated In The Next Fiscal Year [Member]
Reserve Quantities [Line Items]
Amount to be spent on developing proved undeveloped reserves | $	$ 134.0