Business Segment Information (Financial Segment Information By Segment) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Sep. 30, 2012
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	$ 58,495	$ 43,184	$ 212,159	$ 171,275
Segment Assets	6,343,940		6,343,940		5,935,142
Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	440,008	328,861	1,490,688	1,313,593
Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Utility [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	7,630	5,096	65,024	52,725
Segment Assets	2,074,329		2,074,329
Utility [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	141,257	117,240	653,211	622,836
Utility [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	3,305	2,703	14,012	12,643
Pipeline And Storage [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	14,075	12,627	47,803	35,428
Segment Assets	1,281,445		1,281,445
Pipeline And Storage [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	43,055	36,631	132,897	113,976
Pipeline And Storage [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	21,708	22,076	68,216	64,434
Exploration And Production [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	31,734	21,915	86,125	74,422
Segment Assets	2,614,406		2,614,406
Exploration And Production [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	195,213	138,549	518,742	411,449
Exploration And Production [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Energy Marketing [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	963	923	5,741	4,662
Segment Assets	69,106		69,106
Energy Marketing [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	59,128	35,377	182,282	161,822
Energy Marketing [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	446	579	1,080	1,135
Total Reportable Segments [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	54,402	40,561	204,693	167,237
Segment Assets	6,039,286		6,039,286
Total Reportable Segments [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	438,653	327,797	1,487,132	1,310,083
Total Reportable Segments [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	25,459	25,358	83,308	78,212
All Other [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	4,499	2,815	9,449	5,557
Segment Assets	273,048		273,048
All Other [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	1,121	824	2,898	2,784
All Other [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	10,244	4,307	23,622	10,828
Corporate And Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	(406)	(192)	(1,983)	(1,519)
Segment Assets	31,606		31,606
Corporate And Intersegment Eliminations [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	234	240	658	726
Corporate And Intersegment Eliminations [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	$ (35,703)	$ (29,665)	$ (106,930)	$ (89,040)