Segment Information - Revenue from External Customers by Products and Services (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ 4,275,357	$ 4,201,662	$ 3,407,490
Distribution
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	4,096,661	4,031,936	3,238,753
Distribution | Gas sales revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	3,965,371	3,909,893	3,119,194
Distribution | Transportation revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	119,371	110,905	105,554
Distribution | Other gas revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	11,919	11,138	14,005
Distribution | Residential | Gas sales revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	2,638,689	2,492,116	2,117,272
Distribution | Commercial | Gas sales revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	1,112,236	1,126,189	838,382
Distribution | Industrial | Gas sales revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	151,970	224,632	113,171
Distribution | Public authority and other | Gas sales revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	62,476	66,956	50,369
Pipeline and Storage
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ 178,696	$ 169,726	$ 168,737