Business Segment Information (Financial Segment Information By Segment) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenue	$ 552,544	$ 540,905	$ 1,042,791	$ 960,561
Segment Profit: Net Income (Loss)	90,595	91,847	193,256	290,501
Segment Assets	6,278,975		6,278,975		$ 6,036,486
Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	552,544	540,905	1,042,791	960,561
Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Exploration And Production [Member]
Segment Reporting Information [Line Items]
Revenue	146,102		308,978
Segment Profit: Net Income (Loss)	21,873	26,537	60,087	133,235
Segment Assets	1,777,770		1,777,770		1,568,563
Exploration And Production [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	146,102	146,411	308,978	285,552
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]
Revenue	72,339		149,408
Segment Profit: Net Income (Loss)	17,749	22,724	42,851	61,186
Segment Assets	1,861,900		1,861,900		1,848,180
Pipeline And Storage [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	48,421	53,714	102,639	107,025
Pipeline And Storage [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	23,918	23,044	46,769	45,028
Gathering [Member]
Segment Reporting Information [Line Items]
Revenue	29,368		59,058
Segment Profit: Net Income (Loss)	12,690	11,770	26,872	57,169
Segment Assets	554,491		554,491		533,608
Gathering [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	2	(99)	2	71
Gathering [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	29,366	27,832	59,056	51,497
Utility [Member]
Segment Reporting Information [Line Items]
Revenue	303,030		525,687
Segment Profit: Net Income (Loss)	35,589	33,360	61,237	54,353
Segment Assets	2,013,937		2,013,937		1,921,971
Utility [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	298,636	283,778	518,647	470,867
Utility [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	4,394	5,700	7,040	7,882
Energy Marketing [Member]
Segment Reporting Information [Line Items]
Revenue	59,061		111,475
Segment Profit: Net Income (Loss)	544	578	243	1,624
Segment Assets	53,898		53,898		50,971
Energy Marketing [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	59,018	55,644	111,100	94,280
Energy Marketing [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	43	(51)	375	76
Total Reportable Segments [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	88,445	94,969	191,290	307,567
Segment Assets	6,261,996		6,261,996		5,923,293
Total Reportable Segments [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	552,179	539,448	1,041,366	957,795
Total Reportable Segments [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	57,721	56,525	113,240	104,483
All Other [Member]
Segment Reporting Information [Line Items]
Revenue	310		1,316
Segment Profit: Net Income (Loss)	(128)	207	256	(511)
Segment Assets	78,398		78,398		78,109
All Other [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	310	1,232	1,316	2,328
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]
Revenue	(57,666)		(113,131)
Segment Profit: Net Income (Loss)	2,278	(3,329)	1,710	(16,555)
Segment Assets	(61,419)		(61,419)		$ 35,084
Corporate And Intersegment Eliminations [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	55	225	109	438
Corporate And Intersegment Eliminations [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	$ (57,721)	$ (56,525)	$ (113,240)	$ (104,483)