Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Non-Recurring Basis (Details) $ in Thousands	9 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of asset retirement obligation liabilities incurred and acquired	$ 100	$ 1,200
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 | Measurement Input, Remaining Life Per Well
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset retirement obligations, measurement input, period	28 years	27 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	0.0517