Concentration of Credit Risk - Additional Information (Details) $ in Millions	6 Months Ended
	Mar. 31, 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	$ 18.1
Net receivable amount	7.3
Utility Companies And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	94.8
Net receivable amount	84.0
Five Largest Counterparties
Concentration Risk [Line Items]
Accounts receivable	31.5
Net receivable amount	$ 27.3