Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Property, Plant and Equipment
Property, plant and equipment	$ 2,998,915	$ 2,753,499
Distribution
Property, Plant and Equipment
Property, plant and equipment	2,634,191	2,458,080
Transmission
Property, Plant and Equipment
Property, plant and equipment	93,454	90,036
General and Other, Including Contruction in Process
Property, Plant and Equipment
Property, plant and equipment	$ 271,270	$ 205,383