Supplementary Information For Oil And Gas Producing Activities (Principal Sources Of Change In The Standardized Measure Of Discounted Future Net Cash Flows) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Supplementary Information For Oil And Gas Producing Activities Unaudited [Abstract]
Standardized Measure of Discounted Future Net Cash Flows at Beginning of Year	$ 642,528	$ 1,323,034	$ 2,066,878
Sales, Net of Production Costs	(345,075)	(218,444)	(318,753)
Net Changes in Prices, Net of Production Costs	828,187	(1,066,593)	(1,752,843)
Extensions and Discoveries	170,500	47,742	266,159
Changes in Estimated Future Development Costs	8,816	143,752	164,510
Sales of Minerals in Place	(9,849)	(95,849)	(1)
Previously Estimated Development Costs Incurred	101,134	92,840	161,833
Net Change in Income Taxes at Applicable Statutory Rate	(393,353)	387,739	545,442
Revisions of Previous Quantity Estimates	39,078	6,202	(16,573)
Accretion of Discount and Other	71,080	22,105	206,382
Standardized Measure of Discounted Future Net Cash Flows at End of Year	$ 1,113,046	$ 642,528	$ 1,323,034