Business Segment Information - Information By Segment (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2025		Sep. 30, 2024		Sep. 30, 2023
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	$ 2,277,541	[2]	$ 1,944,810		$ 2,173,771	[3]
Intersegment Revenues		0		0		0
Total Revenues		2,277,541		1,944,810		2,173,771
Operation and Maintenance Expense
Upstream General and Administrative Expense	[4]	75,057		70,927		65,833
Lease Operating Expense	[4]	44,848		46,330		40,952
Gathering Operation and Maintenance Expense	[4]	48,389		35,903		33,395
All Other Operation and Maintenance Expense	[4]	389,571		366,858		338,576
Purchased Gas	[4]	213,441		150,062		437,595
Depreciation, Depletion and Amortization	[4]	456,594		457,026		409,573
Impairment of Assets (Significant Non-Cash Item)		141,802	[4]	519,129	[4]	0
Interest Expense	[4]	155,834		138,695		131,886
Interest Income		(6,016)		(8,727)		(11,479)
Income Tax Expense (Benefit)	[4]	175,549		9,742		164,533
Other Expense (Income) Items	[5]	63,968		81,352		86,041
Segment Profit: Net Income (Loss)		518,504		77,513		476,866
Expenditures for Additions to Long-Lived Assets		918,101		942,015		1,123,573
Segment Assets		8,719,104		8,319,770		8,280,260
Total Reportable Segments
Segment Reporting Information [Line Items]
Total Revenues		2,429,366		2,086,370		2,293,897
All Other
Segment Reporting Information [Line Items]
Total Revenues		0		0		0
Corporate
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	0	[2]	0		0	[3]
Corporate and Intersegment Eliminations
Segment Reporting Information [Line Items]
Intersegment Revenues		(151,825)		(141,560)		(120,126)
Total Revenues		(151,825)		(141,560)		(120,126)
Operation and Maintenance Expense
Upstream General and Administrative Expense	[4]	(223)		(221)		(241)
Lease Operating Expense	[4]	(5,817)		(5,723)		(6,902)
Gathering Operation and Maintenance Expense	[4]	(246)		(237)		(255)
All Other Operation and Maintenance Expense	[4]	16,688		12,835		14,035
Purchased Gas	[4]	(145,013)		(133,153)		(110,600)
Depreciation, Depletion and Amortization	[4]	596		473		429
Impairment of Assets (Significant Non-Cash Item)	[4]	0		0
Interest Expense	[4]	(9,813)		(17,839)		(15,309)
Interest Income		3,809		8,703		5,662
Income Tax Expense (Benefit)	[4]	(702)		145		(983)
Other Expense (Income) Items	[5]	(1,518)		(4,955)		(2,464)
Segment Profit: Net Income (Loss)		(9,586)		(1,588)		(3,498)
Expenditures for Additions to Long-Lived Assets		909		970		754
Segment Assets		61,718		(145,727)		(109,647)
Total Reportable Segments | Total Reportable Segments
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	2,277,541	[2]	1,944,810		2,173,771	[3]
Intersegment Revenues		151,825		141,560		120,126
Total Revenues		2,429,366		2,086,370		2,293,897
Operation and Maintenance Expense
Upstream General and Administrative Expense	[4]	75,280		71,148		66,074
Lease Operating Expense	[4]	50,665		52,053		47,854
Gathering Operation and Maintenance Expense	[4]	48,635		36,140		33,650
All Other Operation and Maintenance Expense	[4]	372,883		354,006		324,520
Purchased Gas	[4]	358,454		283,215		548,195
Depreciation, Depletion and Amortization	[4]	455,998		456,553		409,144
Impairment of Assets (Significant Non-Cash Item)	[4]	141,802		519,129
Interest Expense	[4]	165,111		156,160		147,038
Interest Income		(9,814)		(17,430)		(17,141)
Income Tax Expense (Benefit)	[4]	176,496		9,783		165,680
Other Expense (Income) Items	[5]	64,952		85,895		87,988
Segment Profit: Net Income (Loss)		528,904		79,718		480,895
Expenditures for Additions to Long-Lived Assets		917,192		941,045		1,122,819
Segment Assets		8,648,682		8,459,270		8,385,112
Integrated Upstream and Gathering | Total Reportable Segments
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	1,184,136	[2]	976,615		972,346	[3]
Intersegment Revenues		0		0		0
Total Revenues		1,184,136		976,615		972,346
Operation and Maintenance Expense
Upstream General and Administrative Expense	[4]	75,280		71,148		66,074
Lease Operating Expense	[4]	50,665		52,053		47,854
Gathering Operation and Maintenance Expense	[4]	48,635		36,140		33,650
All Other Operation and Maintenance Expense	[4]	16,049		15,529		9,327
Purchased Gas	[4]	0		0		0
Depreciation, Depletion and Amortization	[4]	311,817		316,762		276,867
Impairment of Assets (Significant Non-Cash Item)	[4]	141,802		473,054
Interest Expense	[4]	76,633		74,005		69,306
Interest Income		(1,445)		(3,062)		(3,793)
Income Tax Expense (Benefit)	[4]	121,095		(20,213)		123,924
Other Expense (Income) Items	[5]	18,907		18,240		17,138
Segment Profit: Net Income (Loss)		324,698		(57,041)		331,999
Expenditures for Additions to Long-Lived Assets		605,433		645,600		841,020
Segment Assets		3,701,646		3,614,318		3,710,155
Integrated Upstream and Gathering | Total Reportable Segments | Customer Concentration Risk | One Customer | Revenue Benchmark
Segment Reporting Information [Line Items]
Total Revenues		258,000				208,000
Pipeline and Storage
Segment Reporting Information [Line Items]
Total Revenues		276,131		271,388		259,646
Pipeline and Storage | Total Reportable Segments
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	276,131	[2]	271,388		259,646	[3]
Intersegment Revenues		151,470		141,005		119,545
Total Revenues		427,601		412,393		379,191
Operation and Maintenance Expense
Upstream General and Administrative Expense	[4]	0		0		0
Lease Operating Expense	[4]	0		0		0
Gathering Operation and Maintenance Expense	[4]	0		0		0
All Other Operation and Maintenance Expense	[4]	122,379		116,335		106,654
Purchased Gas	[4]	0		0		0
Depreciation, Depletion and Amortization	[4]	74,480		74,530		70,827
Impairment of Assets (Significant Non-Cash Item)	[4]	0		46,075
Interest Expense	[4]	45,509		47,428		43,499
Interest Income		(6,085)		(8,632)		(7,052)
Income Tax Expense (Benefit)	[4]	39,748		26,045		34,489
Other Expense (Income) Items	[5]	30,613		30,942		30,273
Segment Profit: Net Income (Loss)		120,957		79,670		100,501
Expenditures for Additions to Long-Lived Assets		121,798		110,830		141,877
Segment Assets		2,412,747		2,446,243		2,427,214
Pipeline and Storage | Total Reportable Segments | Customer Concentration Risk | One Customer | Revenue Benchmark
Segment Reporting Information [Line Items]
Total Revenues		16,000				14,000
Utility | Total Reportable Segments
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	817,274	[2]	696,807		941,779	[3]
Intersegment Revenues		355		555		581
Total Revenues		817,629		697,362		942,360
Operation and Maintenance Expense
Upstream General and Administrative Expense	[4]	0		0		0
Lease Operating Expense	[4]	0		0		0
Gathering Operation and Maintenance Expense	[4]	0		0		0
All Other Operation and Maintenance Expense	[4]	234,455		222,142		208,539
Purchased Gas	[4]	358,454		283,215		548,195
Depreciation, Depletion and Amortization	[4]	69,701		65,261		61,450
Impairment of Assets (Significant Non-Cash Item)	[4]	0		0
Interest Expense	[4]	42,969		34,727		34,233
Interest Income		(2,284)		(5,736)		(6,296)
Income Tax Expense (Benefit)	[4]	15,653		3,951		7,267
Other Expense (Income) Items	[5]	15,432		36,713		40,577
Segment Profit: Net Income (Loss)		83,249		57,089		48,395
Expenditures for Additions to Long-Lived Assets		189,961		184,615		139,922
Segment Assets		2,534,289		2,398,709		2,247,743
All Other | All Other
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	0	[2]	0		0	[3]
Intersegment Revenues		0		0		0
Total Revenues		0		0		0
Operation and Maintenance Expense
Upstream General and Administrative Expense	[4]	0		0		0
Lease Operating Expense	[4]	0		0		0
Gathering Operation and Maintenance Expense	[4]	0		0		0
All Other Operation and Maintenance Expense	[4]	0		17		21
Purchased Gas	[4]	0		0		0
Depreciation, Depletion and Amortization	[4]	0		0		0
Impairment of Assets (Significant Non-Cash Item)	[4]	0		0
Interest Expense	[4]	536		374		157
Interest Income		(11)		0		0
Income Tax Expense (Benefit)	[4]	(245)		(186)		(164)
Other Expense (Income) Items	[5]	534		412		517
Segment Profit: Net Income (Loss)		(814)		(617)		(531)
Expenditures for Additions to Long-Lived Assets		0		0		0
Segment Assets		$ 8,704		$ 6,227		$ 4,795

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

[2] Revenue from one customer of the Company’s Integrated Upstream and Gathering segment, exclusive of hedging losses transacted with separate parties, represented approximately $258 million of the Company’s consolidated revenue for the year ended September 30, 2025. This one customer was also a customer of the Company’s Pipeline and Storage segment, accounting for an additional $16 million of the Company’s consolidated revenue for the year ended September 30, 2025.

[3] Revenue from one customer of the Company’s Integrated Upstream and Gathering 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.

[4] The Company considers this line to be a significant expense.

[5] Consists of Property, Franchise and Other Taxes, Non-Service Pension and Post-Retirement Benefits Costs (Credits), Other (Income) Deductions, and Purchased Gas Expense for the Pipeline and Storage segment.