Details of Selected Financial Statement Captions - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024
Balance Sheet Related Disclosures [Abstract]
Storage plant	$ 790,621	$ 708,617
Transmission plant	6,308,557	5,713,831
Distribution plant	19,788,099	17,304,207
General plant	1,102,931	1,019,018
Intangible plant	38,612	38,612
Property, plant and equipment	28,028,820	24,784,285
Construction in progress	1,235,316	1,063,798
Total property, plant and equipment and construction in progress	29,264,136	25,848,083
Less: accumulated depreciation and amortization	(3,971,146)	(3,643,716)
Net property, plant and equipment	25,292,990	22,204,367
Plant acquisition adjustments	$ (21,100)	$ (22,900)