Supplementary Information For Oil And Gas Producing Activities (Narrative) (Details) (USD $)	12 Months Ended							12 Months Ended		12 Months Ended
	Sep. 30, 2012 ft3	Sep. 30, 2011 ft3	Sep. 30, 2010 ft3	Sep. 30, 2011 Covington Area [Member]	Sep. 30, 2012 Marcellus Shale Fields [Member] ft3	Sep. 30, 2011 Marcellus Shale Fields [Member] ft3	Sep. 30, 2010 Marcellus Shale Fields [Member] ft3	Sep. 30, 2012 North Lost Hills Field [Member]	Sep. 30, 2007 North Lost Hills Field [Member]	Sep. 30, 2012 Total PUD Reserve Additions Estimated In 2013 [Member]
Reserve Quantities [Line Items]
Amount spent for developing proved undeveloped reserves	$ 216,600,000	$ 199,200,000	$ 28,900,000
Amount to be spent on developing proved undeveloped reserves										160,000,000
Proved undeveloped (PUD) reserve	115,000,000,000	118,000,000,000
Percentage of PUD reserves to the total proved reserves	33.00%	32.00%	25.00%
New PUD reserve additions	289,000,000,000	212,000,000,000			286,000,000,000	209,000,000,000
Proved Undeveloped Reserve (Volume)	410,000,000,000	295,000,000,000	177,000,000,000		381,000,000,000	253,000,000,000	110,000,000,000
PUD conversions to developed reserves	97,000,000,000	83,000,000,000
Sales of minerals in place		10,000,000,000
Downward PUD revisions	77,000,000,000	2,000,000,000			66,000,000,000
Investment made to convert proved undeveloped reserves to developed reserves	$ 217,000,000	$ 146,000,000		$ 53,000,000
Percentage of PUD reserves booked		33.00%	47.00%
Percentage of undeveloped reserves in total field level reserves								16.00%	44.00%
Conversion of undeveloped proved reserves to developed proved reserves	97,000,000,000	83,000,000,000
Percentage of undeveloped reserves against proved reserves at corporate level								1.00%
Arbitrary discount rate	10.00%