Segment Information - Revenue from External Customers by Products and Services (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ 4,201,662	$ 3,407,490	$ 2,821,137
Distribution
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	4,031,936	3,238,753	2,624,251
Distribution | Gas sales revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	3,909,893	3,119,194	2,503,681
Distribution | Transportation revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	110,905	105,554	97,441
Distribution | Other gas revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	11,138	14,005	23,129
Distribution | Residential | Gas sales revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	2,492,116	2,117,272	1,717,070
Distribution | Commercial | Gas sales revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	1,126,189	838,382	654,963
Distribution | Industrial | Gas sales revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	224,632	113,171	89,641
Distribution | Public authority and other | Gas sales revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	66,956	50,369	42,007
Pipeline and Storage
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ 169,726	$ 168,737	$ 196,886