Supplementary Information For Oil And Gas Producing Activities (Narrative) (Details) $ in Millions, ft³ in Billions	12 Months Ended
	Sep. 30, 2017 USD ($) ft³	Sep. 30, 2016 USD ($) ft³	Sep. 30, 2015 USD ($) ft³	Sep. 30, 2014	Sep. 30, 2013
Reserve Quantities [Line Items]
Amount spent for developing proved undeveloped reserves | $	$ 101.1	$ 92.8	$ 161.8
Proved Undeveloped Reserve (Volume)	612	543	827
Percentage of PUD reserves to the total proved reserves	28.00%	29.00%	35.00%
New PUD reserve additions	269	102
PUD Sales		166
PUD Downward Revisions		14
PUD Upward Revisions	13
PUD conversions to developed reserves	159	110
Proved Undeveloped Reserves, Removed	54	96
Decrease in Proved undeveloped (PUD) reserves		284
Increase in Proved undeveloped (PUD) reserves	69
Investment made to convert proved undeveloped reserves to developed reserves | $	$ 101.0	$ 93.0
Reimbursed Drilling Carry Costs for JDA Partner | $		$ 36.0
Conversion of undeveloped proved reserves to developed proved reserves	147	92
Conversion of PUD to Developed as a Percentage of PUD Reserves Booked at End of Prior Year	27.00%	11.00%	33.00%	51.00%	39.00%
Wells Developed	37
Percent Wells Developed	41.00%
Arbitrary discount rate	10.00%
West Coast Region [Member]
Reserve Quantities [Line Items]
New PUD reserve additions	2
PUD conversions to developed reserves	1
Marcellus Shale Fields [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Volume)	456	542	825
New PUD reserve additions	113	102
PUD conversions to developed reserves	158
Proved Undeveloped Reserves, Removed		74
Wells Developed		31
Percent Wells Developed		28.00%
Utica Shale [Member]
Reserve Quantities [Line Items]
New PUD reserve additions	154
Geneseo Shale [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserves, Removed		23
Will Not Meet 5 Year Requirement for Proved Reserves [Member] | Marcellus Shale Fields [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserves, Removed	36
Change in Development Plans [Member] | Marcellus Shale Fields [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserves, Removed	18
Total PUD Reserve Additions Estimated In The Next Fiscal Year [Member]
Reserve Quantities [Line Items]
Amount to be spent on developing proved undeveloped reserves | $	$ 186.0
Conversion Of Undeveloped Proved Reserves To Developed Proved Reserves Before Revisions [Member]
Reserve Quantities [Line Items]
Conversion of undeveloped proved reserves to developed proved reserves	159	110
Impact of JDA Sales [Member]
Reserve Quantities [Line Items]
Conversion of undeveloped proved reserves to developed proved reserves		207
Conversion of PUD to Developed as a Percentage of PUD Reserves Booked at End of Prior Year		25.00%
Anticipated JDA Sales [Member]
Reserve Quantities [Line Items]
PUD Sales	60