Business Segment Information - (Schedule of Information By Segment) (Details) - USD ($) $ in Thousands		3 Months Ended
		Dec. 31, 2025	Dec. 31, 2024	Sep. 30, 2025
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	$ 651,507	$ 549,482
Intersegment Revenues		0	0
Total Revenues		651,507	549,482
Operation and Maintenance Expense
Upstream General and Administrative Expense	[2]	19,343	19,267
Lease Operating Expense	[2]	16,037	9,064
Gathering Operation and Maintenance Expense	[2]	10,319	6,670
All Other Operation and Maintenance Expense	[2]	97,290	89,725
Purchased Gas	[2]	85,606	65,337
Depreciation, Depletion and Amortization	[2]	122,025	109,370
Impairment of Assets (Significant Non-Cash Item)	[2]	0	141,802
Interest Expense	[2]	43,374	37,743
Interest Income		(2,777)	(1,683)
Income Tax Expense (Benefit)	[2]	59,337	11,182
Other Expense (Income) Items	[3]	19,308	16,019
Segment Profit: Net Income (Loss)		181,645	44,986
Expenditures for Additions to Long-Lived Assets		222,721	192,055
Segment Assets:		9,205,872		$ 8,719,104
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues		689,261	587,429
All Other
Segment Reporting Information [Line Items]
Total Revenues		0	0
Corporate
Segment Reporting Information [Line Items]
Revenues From External Customers	[1],[4]	0	0
Corporate and Intersegment Eliminations
Segment Reporting Information [Line Items]
Intersegment Revenues	[4]	(37,754)	(37,947)
Total Revenues	[4]	(37,754)	(37,947)
Operation and Maintenance Expense
Upstream General and Administrative Expense	[2],[4]	(63)	(59)
Lease Operating Expense	[2],[4]	(789)	(1,587)
Gathering Operation and Maintenance Expense	[2],[4]	(69)	(65)
All Other Operation and Maintenance Expense	[2],[4]	5,652	2,564
Purchased Gas	[2],[4]	(36,679)	(36,136)
Depreciation, Depletion and Amortization	[2],[4]	181	139
Impairment of Assets (Significant Non-Cash Item)	[2],[4]	0	0
Interest Expense	[2],[4]	3,698	(4,228)
Interest Income	[4]	(571)	1,650
Income Tax Expense (Benefit)	[2],[4]	(2,438)	(507)
Other Expense (Income) Items	[3],[4]	913	424
Segment Profit: Net Income (Loss)	[4]	(7,589)	(142)
Expenditures for Additions to Long-Lived Assets	[4]	176	204
Segment Assets:	[4]	121,618		61,718
Total Reportable Segments | Operating Segments
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	651,507	549,482
Intersegment Revenues		37,754	37,947
Total Revenues		689,261	587,429
Operation and Maintenance Expense
Upstream General and Administrative Expense	[2]	19,406	19,326
Lease Operating Expense	[2]	16,826	10,651
Gathering Operation and Maintenance Expense	[2]	10,388	6,735
All Other Operation and Maintenance Expense	[2]	91,638	87,161
Purchased Gas	[2]	122,285	101,473
Depreciation, Depletion and Amortization	[2]	121,844	109,231
Impairment of Assets (Significant Non-Cash Item)	[2]	0	141,802
Interest Expense	[2]	39,540	41,855
Interest Income		(2,196)	(3,333)
Income Tax Expense (Benefit)	[2]	61,812	11,748
Other Expense (Income) Items	[3]	18,362	15,459
Segment Profit: Net Income (Loss)		189,356	45,321
Expenditures for Additions to Long-Lived Assets		222,545	191,851
Segment Assets:		9,075,689		8,648,682
Integrated Upstream and Gathering
Segment Reporting Information [Line Items]
Total Revenues		323,223	252,308
Integrated Upstream and Gathering | Operating Segments
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	323,223	252,308
Intersegment Revenues		0	0
Total Revenues		323,223	252,308
Operation and Maintenance Expense
Upstream General and Administrative Expense	[2]	19,406	19,326
Lease Operating Expense	[2]	16,826	10,651
Gathering Operation and Maintenance Expense	[2]	10,388	6,735
All Other Operation and Maintenance Expense	[2]	3,378	3,867
Purchased Gas	[2]	0	0
Depreciation, Depletion and Amortization	[2]	84,263	73,819
Impairment of Assets (Significant Non-Cash Item)	[2]	0	141,802
Interest Expense	[2]	16,133	19,410
Interest Income		(193)	(679)
Income Tax Expense (Benefit)	[2]	44,111	(6,451)
Other Expense (Income) Items	[3]	4,864	3,460
Segment Profit: Net Income (Loss)		124,047	(19,632)
Expenditures for Additions to Long-Lived Assets		141,849	135,629
Segment Assets:		3,925,191		3,701,646
Pipeline and Storage
Segment Reporting Information [Line Items]
Total Revenues		69,237	68,750
Pipeline and Storage | Operating Segments
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	69,237	68,750
Intersegment Revenues		37,664	37,862
Total Revenues		106,901	106,612
Operation and Maintenance Expense
Upstream General and Administrative Expense	[2]	0	0
Lease Operating Expense	[2]	0	0
Gathering Operation and Maintenance Expense	[2]	0	0
All Other Operation and Maintenance Expense	[2]	27,263	27,034
Purchased Gas	[2]	0	0
Depreciation, Depletion and Amortization	[2]	19,102	18,585
Impairment of Assets (Significant Non-Cash Item)	[2]	0	0
Interest Expense	[2]	11,801	11,729
Interest Income		(964)	(2,006)
Income Tax Expense (Benefit)	[2]	10,366	11,177
Other Expense (Income) Items	[3]	8,114	7,639
Segment Profit: Net Income (Loss)		31,219	32,454
Expenditures for Additions to Long-Lived Assets		37,602	19,792
Segment Assets:		2,504,541		2,412,747
Utility
Segment Reporting Information [Line Items]
Total Revenues		259,047	228,424
Utility | Operating Segments
Segment Reporting Information [Line Items]
Revenues From External Customers	[1]	259,047	228,424
Intersegment Revenues		90	85
Total Revenues		259,137	228,509
Operation and Maintenance Expense
Upstream General and Administrative Expense	[2]	0	0
Lease Operating Expense	[2]	0	0
Gathering Operation and Maintenance Expense	[2]	0	0
All Other Operation and Maintenance Expense	[2]	60,997	56,260
Purchased Gas	[2]	122,285	101,473
Depreciation, Depletion and Amortization	[2]	18,479	16,827
Impairment of Assets (Significant Non-Cash Item)	[2]	0	0
Interest Expense	[2]	11,606	10,716
Interest Income		(1,039)	(648)
Income Tax Expense (Benefit)	[2]	7,335	7,022
Other Expense (Income) Items	[3]	5,384	4,360
Segment Profit: Net Income (Loss)		34,090	32,499
Expenditures for Additions to Long-Lived Assets		43,094	36,430
Segment Assets:		2,645,957		2,534,289
All Other | All Other
Segment Reporting Information [Line Items]
Revenues From External Customers	[1],[4]	0	0
Intersegment Revenues	[4]	0	0
Total Revenues	[4]	0	0
Operation and Maintenance Expense
Upstream General and Administrative Expense	[2],[4]	0	0
Lease Operating Expense	[2],[4]	0	0
Gathering Operation and Maintenance Expense	[2],[4]	0	0
All Other Operation and Maintenance Expense	[2],[4]	0	0
Purchased Gas	[2],[4]	0	0
Depreciation, Depletion and Amortization	[2],[4]	0	0
Impairment of Assets (Significant Non-Cash Item)	[2],[4]	0	0
Interest Expense	[2],[4]	136	116
Interest Income	[4]	(10)	0
Income Tax Expense (Benefit)	[2],[4]	(37)	(59)
Other Expense (Income) Items	[3],[4]	33	136
Segment Profit: Net Income (Loss)	[4]	(122)	(193)
Expenditures for Additions to Long-Lived Assets	[4]	0	$ 0
Segment Assets:	[4]	$ 8,565		$ 8,704

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

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

[3] 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.

[4] Corporate and All Other categories primarily represent other non-segment business activities and eliminating entries.