Details of Selected Consolidated Balance Sheet Captions - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020
Balance Sheet Related Disclosures [Abstract]
Storage plant	$ 539,972	$ 530,985
Transmission plant	3,725,347	3,459,765
Distribution plant	12,085,654	10,680,495
General plant	868,962	829,624
Intangible plant	38,612	38,297
Property, plant and equipment	17,258,547	15,539,166
Construction in progress	626,551	418,055
Total property, plant and equipment and construction in progress	17,885,098	15,957,221
Less: accumulated depreciation and amortization	(2,821,128)	(2,601,874)
Net property, plant and equipment	15,063,970	13,355,347
Plant acquisition adjustments	$ (28,500)	$ (37,800)