Fair Value Measurements (Narrative) (Details) - USD ($)	Sep. 30, 2020	Sep. 30, 2019		Sep. 30, 2018	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Hedging collateral deposits	$ 0	$ 6,832,000	[1]	$ 3,441,000	$ 1,741,000
Level 1 or Level 2 Transfers	0	0
Derivative Financial Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Level 3 Fair Value	0	0
Level 1 or Level 2 Transfers	$ 0	0
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Hedging collateral deposits		6,832,000
Fair Value, Inputs, Level 1 [Member] | Futures [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Hedging collateral deposits		6,800,000
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Hedging collateral deposits		$ 0

[1] Netting Adjustments represent the impact of legally-enforceable master netting arrangements that allow the Company to net gain and loss positions held with the same counterparties. The net asset or net liability for each counterparty is recorded as an asset or liability on the Company’s balance sheet.