Other Operating Income (Expense), Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Component of Operating Income [Abstract]
Non-tariff service income	$ 1,491	$ 2,633	$ 4,760
Environmental matters	6,155	(2,918)	1,152
Construction service income	0	0	2,175
Sale of HVAC Business	0	0	1,065
Net interest on PGC overcollection	(130)	(1,740)	(606)
Other, net	813	25	323
Total other operating income (expense), net	$ 8,329	$ (2,000)	$ 8,869