Details of Selected Consolidated Balance Sheet Captions - Property, Plant and Equipment Detail Table (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Statement of Financial Position [Abstract]
Storage plant	$ 414,857	$ 369,510
Transmission plant	2,851,423	2,521,671
Distribution plant	8,141,733	7,306,021
General plant	771,355	765,728
Intangible plant	38,280	38,980
Property, plant and equipment	12,217,648	11,001,910
Construction in progress	349,725	299,394
Total property, plant and equipment and construction in progress	12,567,373	11,301,304
Less: accumulated depreciation and amortization	(2,196,226)	(2,042,122)
Net property, plant and equipment	10,371,147	9,259,182
Plant acquisition adjustments	$ (55,500)	$ (64,100)