Concentration of Credit Risk - Additional Information (Details) $ in Millions	3 Months Ended
	Dec. 31, 2018 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	$ 26.0
Net receivable amount	13.2
Utility Companies And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	121.9
Net receivable amount	102.5
Five Largest Counterparties
Concentration Risk [Line Items]
Accounts receivable	41.8
Net receivable amount	$ 35.0