Business Segment Information (Financial Segment Information By Segment) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013	Sep. 30, 2013
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	$ 95,211	$ 85,720	$ 177,463	$ 153,664
Segment Assets	6,567,195		6,567,195		6,218,347
Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	756,242	597,826	1,306,314	1,050,680
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)	35,545	34,516	59,760	57,394
Segment Assets	2,019,438		2,019,438		1,870,587
Utility [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	377,647	303,389	608,100	511,953
Utility [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	8,204	6,396	12,911	10,707
Pipeline And Storage [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	21,372	16,796	40,510	33,728
Segment Assets	1,283,654		1,283,654		1,246,027
Pipeline And Storage [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	53,571	46,383	104,784	89,842
Pipeline And Storage [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	22,235	23,712	42,974	46,509
Exploration And Production [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	24,390	27,711	55,487	54,391
Segment Assets	2,821,355		2,821,355		2,746,233
Exploration And Production [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	199,561	168,080	392,607	323,529
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)	3,765	4,283	5,369	4,778
Segment Assets	110,285		110,285		67,267
Energy Marketing [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	124,439	78,989	197,598	123,154
Energy Marketing [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	5	208	260	634
Gathering [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	7,324	3,093	13,471	5,035
Segment Assets	247,286		247,286		203,323
Gathering [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	195	324	429	526
Gathering [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	15,452	7,898	29,802	13,377
Total Reportable Segments [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	92,396	86,399	174,597	155,326
Segment Assets	6,482,018		6,482,018		6,133,437
Total Reportable Segments [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	755,413	597,165	1,303,518	1,049,004
Total Reportable Segments [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	45,896	38,214	85,947	71,227
All Other [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	278	(29)	954	(85)
Segment Assets	95,849		95,849		95,793
All Other [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	597	437	2,298	1,252
All Other [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Corporate And Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	2,537	(650)	1,912	(1,577)
Segment Assets	(10,672)		(10,672)		(10,883)
Corporate And Intersegment Eliminations [Member] | Revenue From External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	232	224	498	424
Corporate And Intersegment Eliminations [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	$ (45,896)	$ (38,214)	$ (85,947)	$ (71,227)