Business Segment Information (Segment Information By Segment) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting Information [Line Items]
Revenue from External Customers	$ 552,309	$ 660,881	$ 984,732	$ 1,111,829
Intersegment Revenues	0	0	0	0
Segment Profit: Net Income (Loss)	67,392	115,611	128,091	174,154
Utility [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers	296,786	361,745	505,596	604,587
Intersegment Revenues	5,551	6,635	9,940	11,205
Segment Profit: Net Income (Loss)	28,275	33,081	47,628	56,071
Pipeline And Storage [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers	42,120	39,669	77,345	73,182
Intersegment Revenues	21,294	20,632	42,359	40,514
Segment Profit: Net Income (Loss)	12,841	10,955	22,801	19,533
Exploration And Production [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers	136,926	137,430	272,899	257,598
Intersegment Revenues	0	0	0	0
Segment Profit: Net Income (Loss)	22,192	33,299	52,507	60,672
Energy Marketing [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers	75,223	121,321	126,445	174,973
Intersegment Revenues	269	0	556	0
Segment Profit: Net Income (Loss)	3,310	6,299	3,739	7,231
Total Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers	551,055	660,165	982,285	1,110,340
Intersegment Revenues	27,114	27,267	52,855	51,719
Segment Profit: Net Income (Loss)	66,618	83,634	126,675	143,507
All Other [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers	1,023	472	1,960	1,021
Intersegment Revenues	3,159	2,538	6,520	4,216
Segment Profit: Net Income (Loss)	1,339	32,181	2,743	31,606
Corporate And Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers	231	244	487	468
Intersegment Revenues	(30,273)	(29,805)	(59,375)	(55,935)
Segment Profit: Net Income (Loss)	$ (565)	$ (204)	$ (1,327)	$ (959)