Details of Selected Consolidated Balance Sheet Captions - Property, Plant and Equipment Detail Table (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Statement of Financial Position [Abstract]
Storage plant	$ 431,286	$ 414,857
Transmission plant	3,157,316	2,851,423
Distribution plant	9,333,011	8,141,733
General plant	799,095	771,355
Intangible plant	38,191	38,280
Property, plant and equipment	13,758,899	12,217,648
Construction in progress	421,694	349,725
Total property, plant and equipment and construction in progress	14,180,593	12,567,373
Less: accumulated depreciation and amortization	(2,392,924)	(2,196,226)
Net property, plant and equipment	11,787,669	10,371,147
Plant acquisition adjustments	$ (46,700)	$ (55,500)