Business Segment Information (Financial Segment Information By Segment) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenue	$ 551,115	$ 491,095	$ 992,275	$ 935,283
Segment Profit: Net Income (Loss)	112,436	(106,068)	190,210	(19,477)
Segment Assets	7,115,652		7,115,652		$ 6,964,935
Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	551,115	491,095	992,275	935,283
Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Exploration And Production [Member]
Segment Reporting Information [Line Items]
Revenue	220,187	155,560	411,582	321,499
Segment Profit: Net Income (Loss)	36,822	(175,275)	7,199	(151,299)
Segment Assets	2,023,147		2,023,147		1,979,028
Exploration And Production [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	220,187	155,560	411,582	321,499
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	86,704	79,245	174,469	151,465
Segment Profit: Net Income (Loss)	24,928	22,087	49,112	40,192
Segment Assets	2,240,140		2,240,140		2,204,971
Pipeline And Storage [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	59,314	51,919	118,623	100,888
Pipeline And Storage [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	27,390	27,326	55,846	50,577
Gathering [Member]
Segment Reporting Information [Line Items]
Revenue	50,262	35,267	97,270	70,055
Segment Profit: Net Income (Loss)	20,700	19,898	41,250	35,842
Segment Assets	855,507		855,507		945,199
Gathering [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	671	0	1,021	0
Gathering [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	49,591	35,267	96,249	70,055
Utility [Member]
Segment Reporting Information [Line Items]
Revenue	270,881	254,493	459,881	451,318
Segment Profit: Net Income (Loss)	32,044	31,499	55,081	58,082
Segment Assets	2,151,304		2,151,304		2,067,852
Utility [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	270,784	250,556	459,684	445,465
Utility [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	97	3,937	197	5,853
Total Reportable Segments [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	114,494	(101,791)	152,642	(17,183)
Segment Assets	7,270,098		7,270,098		7,197,050
Total Reportable Segments [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	550,956	458,035	990,910	867,852
Total Reportable Segments [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	77,078	66,530	152,292	126,485
All Other [Member]
Segment Reporting Information [Line Items]
Revenue	65	33,004	1,195	67,417
Segment Profit: Net Income (Loss)	(983)	1,169	36,577	1,540
Segment Assets	27,391		27,391		113,571
All Other [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	64	32,925	1,175	67,161
All Other [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	1	79	20	256
Corporate And Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(76,984)	(66,474)	(152,122)	(126,471)
Segment Profit: Net Income (Loss)	(1,075)	(5,446)	991	(3,834)
Segment Assets	(181,837)		(181,837)		$ (345,686)
Corporate And Intersegment Eliminations [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	95	135	190	270
Corporate And Intersegment Eliminations [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	$ (77,079)	$ (66,609)	$ (152,312)	$ (126,741)