Business Segment Information (Financial Segment Information By Segment) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	$ 91,847	$ 89,284	$ 290,501	$ 178,191
Segment Assets	5,914,573		5,914,573		$ 6,103,320
Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	540,905	522,075	960,561	944,576
Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Exploration And Production [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	26,537	33,769	133,235	68,849
Segment Assets	1,540,251		1,540,251		1,407,152
Exploration And Production [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	146,411	159,553	285,552	320,485
Exploration And Production [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Pipeline And Storage [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	22,724	19,256	61,186	38,624
Segment Assets	1,764,554		1,764,554		1,929,788
Pipeline And Storage [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	53,714	53,163	107,025	106,164
Pipeline And Storage [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	23,044	22,592	45,028	44,746
Gathering [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	11,770	10,285	57,169	21,266
Segment Assets	517,335		517,335		580,051
Gathering [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	(99)	26	71	52
Gathering [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	27,832	27,936	51,497	55,776
Utility [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	33,360	25,581	54,353	46,755
Segment Assets	1,963,421		1,963,421		2,013,123
Utility [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	283,778	257,949	470,867	428,919
Utility [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	5,700	6,096	7,882	7,922
Energy Marketing [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	578	905	1,624	2,687
Segment Assets	61,166		61,166		60,937
Energy Marketing [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	55,644	50,940	94,280	87,750
Energy Marketing [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(51)	16	76	35
Total Reportable Segments [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	94,969	89,796	307,567	178,181
Segment Assets	5,846,727		5,846,727		5,991,051
Total Reportable Segments [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	539,448	521,631	957,795	943,370
Total Reportable Segments [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	56,525	56,640	104,483	108,479
All Other [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	207	(221)	(511)	(400)
Segment Assets	77,252		77,252		76,861
All Other [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	1,232	218	2,328	772
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)	(3,329)	(291)	(16,555)	410
Segment Assets	(9,406)		(9,406)		$ 35,408
Corporate And Intersegment Eliminations [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	225	226	438	434
Corporate And Intersegment Eliminations [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	$ (56,525)	$ (56,640)	$ (104,483)	$ (108,479)