Business Segment Information - Information By Segment (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2023		Sep. 30, 2022		Sep. 30, 2021
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	$ 2,173,771	[2]	$ 2,186,046	[3]	$ 1,742,659
Intersegment Revenues		0		0		0
Interest Income		11,479		6,111		4,388
Interest Expense		131,886		130,357		146,357
Depreciation, Depletion and Amortization		409,573		369,790		335,303
Income Tax Expense (Benefit)		164,533		116,629		114,682
Significant Item: Gain on Sale of Assets		0		12,736		51,066
Significant Non-Cash Item: Impairment of Oil and Gas Producing Properties		0		0		76,152
Segment Profit: Net Income (Loss)		476,866		566,021		363,647
Expenditures for Additions to Long-Lived Assets		1,123,573		829,388		769,911
Segment Assets		8,280,260		7,896,262		7,464,825
Operating Revenues		2,173,771		2,186,046		1,742,659
Total Reportable Segments
Segment Reporting Information [Line Items]
Operating Revenues		2,510,323		2,500,572
All Other
Segment Reporting Information [Line Items]
Segment Assets		4,795
Operating Revenues		0		6
Corporate
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	0	[2]	165	[3]	356
Corporate and Intersegment Eliminations
Segment Reporting Information [Line Items]
Intersegment Revenues		(336,552)		(314,697)		(299,688)
Interest Income		(5,662)		(1,024)		486
Interest Expense		(15,309)		(6,143)		(3,569)
Depreciation, Depletion and Amortization		429		183		179
Income Tax Expense (Benefit)		(983)		(4,429)		(1,821)
Significant Item: Gain on Sale of Assets				0		0
Significant Non-Cash Item: Impairment of Oil and Gas Producing Properties						0
Segment Profit: Net Income (Loss)		(3,498)		(12,650)		(3,065)
Expenditures for Additions to Long-Lived Assets		754		1,212		673
Segment Assets		(126,633)		(186,281)		(107,405)
Operating Revenues		(336,552)		(314,532)
Total Reportable Segments | Total Reportable Segments
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	2,173,771	[2]	2,185,881	[3]	1,741,130
Intersegment Revenues		336,552		314,691		299,639
Interest Income		17,141		7,132		3,672
Interest Expense		147,038		136,496		149,926
Depreciation, Depletion and Amortization		409,144		369,607		334,730
Income Tax Expense (Benefit)		165,680		121,055		105,065
Significant Item: Gain on Sale of Assets				12,736		0
Significant Non-Cash Item: Impairment of Oil and Gas Producing Properties						76,152
Segment Profit: Net Income (Loss)		480,895		578,680		329,067
Expenditures for Additions to Long-Lived Assets		1,122,819		828,176		769,238
Segment Assets		8,402,098		8,080,507		7,568,084
Exploration and Production | Total Reportable Segments
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	958,455	[2]	1,010,464	[3]	836,697
Intersegment Revenues		0		0		0
Interest Income		3,259		1,929		211
Interest Expense		54,317		53,401		69,662
Depreciation, Depletion and Amortization		241,142		208,148		182,492
Income Tax Expense (Benefit)		87,796		43,898		33,370
Significant Item: Gain on Sale of Assets				12,736		0
Significant Non-Cash Item: Impairment of Oil and Gas Producing Properties						76,152
Segment Profit: Net Income (Loss)		232,275		306,064		101,916
Expenditures for Additions to Long-Lived Assets		737,725		565,791		381,408
Segment Assets		2,814,218		2,507,541		2,286,058
Operating Revenues		958,455		1,010,464
Exploration and Production | Total Reportable Segments | Customer Concentration Risk | Three Customers | Revenue Benchmark
Segment Reporting Information [Line Items]
Operating Revenues				850,000
Exploration and Production | Total Reportable Segments | Customer Concentration Risk | One Customer | Revenue Benchmark
Segment Reporting Information [Line Items]
Operating Revenues		208,000
Pipeline and Storage | Total Reportable Segments
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	259,646	[2]	265,415	[3]	234,397
Intersegment Revenues		119,545		111,629		109,160
Interest Income		7,052		2,275		1,085
Interest Expense		43,499		42,492		40,976
Depreciation, Depletion and Amortization		70,827		67,701		62,431
Income Tax Expense (Benefit)		34,489		35,043		28,812
Significant Item: Gain on Sale of Assets				0		0
Significant Non-Cash Item: Impairment of Oil and Gas Producing Properties						0
Segment Profit: Net Income (Loss)		100,501		102,557		92,542
Expenditures for Additions to Long-Lived Assets		141,877		95,806		252,316
Segment Assets		2,427,214		2,394,697		2,296,030
Operating Revenues		379,191		377,044
Pipeline and Storage | Total Reportable Segments | Customer Concentration Risk | Three Customers | Revenue Benchmark
Segment Reporting Information [Line Items]
Operating Revenues				15,000
Pipeline and Storage | Total Reportable Segments | Customer Concentration Risk | One Customer | Revenue Benchmark
Segment Reporting Information [Line Items]
Operating Revenues		14,000
Gathering | Total Reportable Segments
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	13,891	[2]	12,086	[3]	3,116
Intersegment Revenues		216,426		202,757		190,148
Interest Income		534		198		259
Interest Expense		14,989		16,488		17,493
Depreciation, Depletion and Amortization		35,725		33,998		32,350
Income Tax Expense (Benefit)		36,128		24,949		28,876
Significant Item: Gain on Sale of Assets				0		0
Significant Non-Cash Item: Impairment of Oil and Gas Producing Properties						0
Segment Profit: Net Income (Loss)		99,724		101,111		80,274
Expenditures for Additions to Long-Lived Assets		103,295		55,546		34,669
Segment Assets		912,923		878,796		837,729
Operating Revenues		230,317		214,843
Utility | Total Reportable Segments
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	941,779	[2]	897,916	[3]	666,920
Intersegment Revenues		581		305		331
Interest Income		6,296		2,730		2,117
Interest Expense		34,233		24,115		21,795
Depreciation, Depletion and Amortization		61,450		59,760		57,457
Income Tax Expense (Benefit)		7,267		17,165		14,007
Significant Item: Gain on Sale of Assets				0		0
Significant Non-Cash Item: Impairment of Oil and Gas Producing Properties						0
Segment Profit: Net Income (Loss)		48,395		68,948		54,335
Expenditures for Additions to Long-Lived Assets		139,922		111,033		100,845
Segment Assets		2,247,743		2,299,473		2,148,267
Operating Revenues		942,360		898,221
All Other | All Other
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	0	[2]	0	[3]	1,173
Intersegment Revenues		0		6		49
Interest Income		0		3		230
Interest Expense		157		4		0
Depreciation, Depletion and Amortization		0		0		394
Income Tax Expense (Benefit)		(164)		3		11,438
Significant Item: Gain on Sale of Assets				0		51,066
Significant Non-Cash Item: Impairment of Oil and Gas Producing Properties						0
Segment Profit: Net Income (Loss)		(531)		(9)		37,645
Expenditures for Additions to Long-Lived Assets		$ 0		0		0
Segment Assets				$ 2,036		$ 4,146

[1] All Revenue from External Customers originated in the United States.

[2] Revenue from one customer of the Company's Exploration and Production segment, exclusive of hedging losses transacted with separate parties, represented approximately $208 million of the Company's consolidated revenue for the year ended September 30, 2023. This one customer was also a customer of the Company's Pipeline and Storage segment, accounting for an additional $14 million of the Company's consolidated revenue for the year ended September 30, 2023.

[3] Revenues from three customers of the Company's Exploration and Production segment, exclusive of hedging losses transacted with separate parties, represented approximately $850 million of the Company's consolidated revenue for the year ended September 30, 2022. These three customers were also customers of the Company's Pipeline and Storage segment, accounting for an additional $15 million of the Company's consolidated revenue for the year ended September 30, 2022.