Debt - Energy Services (Details) - Energy Services - USD ($)	Mar. 06, 2020	Sep. 30, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 260,000,000	$ 200,000,000
Line of Credit | Energy Services Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 260,000,000
Line of credit facility accordion feature increase limit	$ 325,000,000
Variable interest rate floor (as a percentage)	1.00%
Line of Credit | Energy Services Credit Agreement | Minimum
Debt Instrument [Line Items]
Variable interest rate floor (as a percentage)	0.75%
Line of Credit | Energy Services Credit Agreement | Maximum
Debt Instrument [Line Items]
Variable interest rate floor (as a percentage)	2.75%
Line of Credit | Energy Services Credit Agreement | Federal Funds Effective Swap Rate
Debt Instrument [Line Items]
Basis spread on variable rate (as a percentage)	0.50%
Line of Credit | Energy Services Credit Agreement | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate (as a percentage)	1.00%
Line of Credit | Energy Services Credit Agreement | Alternate Rate And Adjusted L I B O R | Minimum
Debt Instrument [Line Items]
Variable interest rate floor (as a percentage)	1.50%
Line of Credit | Energy Services Credit Agreement | Alternate Rate And Adjusted L I B O R | Maximum
Debt Instrument [Line Items]
Variable interest rate floor (as a percentage)	2.50%
Letter of Credit | Line of Credit | Energy Services Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 50,000,000