Concentrations of Credit Risk - Additional Information (Details) customer in Millions, $ in Millions	3 Months Ended
	Dec. 31, 2019 USD ($) customer State 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	2
Number of outstanding credit guaranteed | counterparty	1
Number of outstanding credit supported | counterparty	1
Energy Producers And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	$ 20.9
Net receivable amount	11.7
Utility Companies And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	84.2
Net receivable amount	75.4
Five Largest Counterparties
Concentration Risk [Line Items]
Accounts receivable	33.6
Net receivable amount	$ 30.7
Gas Utility
Concentration Risk [Line Items]
Number of customer serves | customer	1.7
Number of state serves across multiple rate classes | State	3