Business Segment Information (Financial Segment Information By Segment) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenue	$ 491,095	$ 552,544	$ 935,283	$ 1,042,791
Segment Profit: Net Income (Loss)	(106,068)	90,595	(19,477)	193,256
Segment Assets	6,708,659		6,708,659		$ 6,462,157
Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	491,095	552,544	935,283	1,042,791
Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Exploration And Production [Member]
Segment Reporting Information [Line Items]
Revenue	155,560	146,102	321,499	308,978
Segment Profit: Net Income (Loss)	(175,275)	21,873	(151,299)	60,087
Segment Assets	2,014,520		2,014,520		1,972,776
Exploration And Production [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	155,560	146,102	321,499	308,978
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	79,245	72,339	151,465	149,408
Segment Profit: Net Income (Loss)	22,087	17,749	40,192	42,851
Segment Assets	1,916,849		1,916,849		1,893,514
Pipeline And Storage [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	51,919	48,421	100,888	102,639
Pipeline And Storage [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	27,326	23,918	50,577	46,769
Gathering [Member]
Segment Reporting Information [Line Items]
Revenue	35,267	29,368	70,055	59,058
Segment Profit: Net Income (Loss)	19,898	12,690	35,842	26,872
Segment Assets	576,589		576,589		547,995
Gathering [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	0	2	0	2
Gathering [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	35,267	29,366	70,055	59,056
Utility [Member]
Segment Reporting Information [Line Items]
Revenue	254,493	303,030	451,318	525,687
Segment Profit: Net Income (Loss)	31,499	35,589	58,082	61,237
Segment Assets	2,049,424		2,049,424		1,991,338
Utility [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	250,556	298,636	445,465	518,647
Utility [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	3,937	4,394	5,853	7,040
Total Reportable Segments [Member]
Segment Reporting Information [Line Items]
Segment Profit: Net Income (Loss)	(101,791)	87,901	(17,183)	191,047
Segment Assets	6,557,382		6,557,382		6,405,623
Total Reportable Segments [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	458,035	493,161	867,852	930,266
Total Reportable Segments [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	66,530	57,678	126,485	112,865
All Other [Member]
Segment Reporting Information [Line Items]
Revenue	33,004	59,371	67,417	112,791
Segment Profit: Net Income (Loss)	1,169	416	1,540	499
Segment Assets	134,926		134,926		122,241
All Other [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	32,925	59,328	67,161	112,416
All Other [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	79	43	256	375
Corporate And Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(66,474)	(57,666)	(126,471)	(113,131)
Segment Profit: Net Income (Loss)	(5,446)	2,278	(3,834)	1,710
Segment Assets	16,351		16,351		$ (65,707)
Corporate And Intersegment Eliminations [Member] | Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	135	55	270	109
Corporate And Intersegment Eliminations [Member] | Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	$ (66,609)	$ (57,721)	$ (126,741)	$ (113,240)