Summary of Significant Accounting Policies - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Accounting Policies [Abstract]
Interest expense	$ 6,800,000	$ 2,500,000	$ 2,800,000
Composite depreciation rate for regulated property, plant and equipment	3.20%	3.10%	3.20%
Asset retirement obligations	$ 12,900,000	$ 12,800,000
Asset retirement costs	7,500,000	$ 7,800,000
Cash held in margin accounts	0
Cumulative effect of accounting change	22,900,000		$ 14,527,000
Expected cumulative-effect adjustment	$ 8,000,000