Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Property, Plant and Equipment
Property, plant and equipment	$ 3,616,289	$ 3,285,329
Distribution
Property, Plant and Equipment
Property, plant and equipment	3,106,599	2,835,339
Transmission
Property, Plant and Equipment
Property, plant and equipment	97,075	96,430
Computer equipment and software
Property, Plant and Equipment
Property, plant and equipment	127,201	113,017
Construction in process
Property, Plant and Equipment
Property, plant and equipment	130,893	112,563
General and other
Property, Plant and Equipment
Property, plant and equipment	$ 154,521	$ 127,980