Business Segment Information (Financial Segment Information By Segment) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Revenue from External Customers	$ 597,826	$ 552,309	$ 1,050,680	$ 984,732
Intersegment Revenues	0	0	0	0
Segment Profit: Net Income (Loss)	85,720	67,392	153,664	128,091
Utility [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers	303,389	296,786	511,953	505,596
Intersegment Revenues	6,396	5,551	10,707	9,940
Segment Profit: Net Income (Loss)	34,516	28,275	57,394	47,628
Pipeline And Storage [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers	46,383	42,120	89,842	77,345
Intersegment Revenues	23,712	21,294	46,509	42,359
Segment Profit: Net Income (Loss)	16,796	12,841	33,728	22,801
Exploration And Production [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers	168,080	136,926	323,529	272,899
Intersegment Revenues	0	0	0	0
Segment Profit: Net Income (Loss)	27,711	22,192	54,391	52,507
Energy Marketing [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers	78,989	75,223	123,154	126,445
Intersegment Revenues	208	269	634	556
Segment Profit: Net Income (Loss)	4,283	3,310	4,778	3,739
Total Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers	596,841	551,055	1,048,478	982,285
Intersegment Revenues	30,316	27,114	57,850	52,855
Segment Profit: Net Income (Loss)	83,306	66,618	150,291	126,675
All Other [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers	761	1,023	1,778	1,960
Intersegment Revenues	7,898	3,159	13,377	6,520
Segment Profit: Net Income (Loss)	3,064	1,339	4,950	2,743
Corporate And Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers	224	231	424	487
Intersegment Revenues	(38,214)	(30,273)	(71,227)	(59,375)
Segment Profit: Net Income (Loss)	$ (650)	$ (565)	$ (1,577)	$ (1,327)