Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Non-Recurring Basis (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of asset retirement obligation liabilities incurred and acquired	$ 53	$ 889
Valuation Technique, Discounted Cash Flow | Measurement Input, Estimated Plug And Abandonment Cost Per Well For All Oil And Natural Gas Wells
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset retirement obligations, measurement input	50	50
Valuation Technique, Discounted Cash Flow | Measurement Input, Estimated Plug And Abandonment Cost Per Well For Disposal Wells
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset retirement obligations, measurement input	52	52
Valuation Technique, Discounted Cash Flow | Remaining Life Per Well
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset retirement obligations, measurement input, period	27 years	12 years
Valuation Technique, Discounted Cash Flow | Measurement Input, Average Credit-Adjusted Risk Free Interest Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset retirement obligations, measurement input	0.0517	0.0834