Concentration of Credit Risk - Additional Information (Details) $ in Millions	9 Months Ended
	Jun. 30, 2019 USD ($) subsegment counterparty
Concentration Risk [Line Items]
Number of groups with potential to affect overall exposure | subsegment	3
Number of large counterparties for which credit risk is disclosed | counterparty	5
Number of counterparties with investment-grade rating | counterparty	4
Energy Producers And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	$ 12.5
Net receivable amount	5.8
Utility Companies And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	99.0
Net receivable amount	89.3
Five Largest Counterparties
Concentration Risk [Line Items]
Accounts receivable	48.3
Net receivable amount	$ 44.9