Quarterly Financial Data (Schedule Of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Operating Revenues	$ 286,937	$ 348,369	$ 522,075	$ 422,500	$ 292,472		$ 335,617		$ 449,132		$ 375,195		$ 1,579,881	$ 1,452,416	$ 1,760,913
Operating Income (Loss)	87,395	123,354	169,957	172,139	81,244		45,162		(237,000)		(305,924)		552,845	(416,518)	(611,053)
Net Income (Loss) Available for Common Stock	$ 45,577	$ 59,714	$ 89,283	$ 88,908	$ 37,553	[1]	$ 8,286	[2]	$ (147,688)	[3]	$ (189,109)	[4]	$ 283,482	$ (290,958)	$ (379,427)
Earnings (Loss) per Common Share, Basic	$ 0.53	$ 0.70	$ 1.05	$ 1.04	$ 0.44		$ 0.10		$ (1.74)		$ (2.23)		$ 3.32	$ (3.43)	$ (4.50)
Earnings (Loss) per Common Share, Diluted	$ 0.53	$ 0.69	$ 1.04	$ 1.04	$ 0.44		$ 0.10		$ (1.74)		$ (2.23)		$ 3.30	$ (3.43)	$ (4.50)
Impairment of Oil and Gas Producing Properties					$ 32,700		$ 82,700		$ 397,400		$ 435,500		$ 0	$ 948,307	$ 1,126,257
Impairment of Oil and Gas Producing Properties After Tax					$ 19,000		$ 47,900		$ 230,500		$ 252,600

[1] Includes a non-cash $32.7 million impairment charge ($19.0 million after tax) associated with the Exploration and Production segment's oil and gas producing properties.

[2] Includes a non-cash $82.7 million impairment charge ($47.9 million after tax) associated with the Exploration and Production segment's oil and gas producing properties.

[3] Includes a non-cash $397.4 million impairment charge ($230.5 million after tax) associated with the Exploration and Production segment's oil and gas producing properties.

[4] Includes a non-cash $435.5 million impairment charge ($252.6 million after tax) associated with the Exploration and Production segment's oil and gas producing properties.