Summary of Significant Accounting Policies - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Alternative Revenue Program [Line Items]
Composite depreciation rate for regulated property, plant and equipment	3.00%	3.10%	3.20%
Asset retirement obligations	$ 20,300,000	$ 17,100,000
Asset retirement costs	14,400,000	11,300,000
Cash held in margin accounts	$ 0	$ 0
Pipeline and Storage
Alternative Revenue Program [Line Items]
Regulatory mechanism threshold	75.00%
Rate Case Revenue Benchmark	$ 69,400,000