Details of Selected Consolidated Balance Sheet Captions - Property, Plant and Equipment Detail Table (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Storage plant	$ 530,985	$ 431,286
Transmission plant	3,459,765	3,157,316
Distribution plant	10,680,495	9,333,011
General plant	829,624	799,095
Intangible plant	38,297	38,191
Property, plant and equipment	15,539,166	13,758,899
Construction in progress	418,055	421,694
Total property, plant and equipment and construction in progress	15,957,221	14,180,593
Less: accumulated depreciation and amortization	(2,601,874)	(2,392,924)
Net property, plant and equipment	13,355,347	11,787,669
Plant acquisition adjustments	$ (37,800)	$ (46,700)