Derivatives and Hedging-Disclosures and Fair Value Measurements - Company's Commodity Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 - Commodity Contract - USD ($) $ in Thousands	Mar. 31, 2022	Sep. 30, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	$ 40,432	$ 31,383
Derivative Liabilities, commodity contracts	(726)	(3,601)
Fair liability and fair asset value of derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	38,503	29,360
Derivative Liabilities, commodity contracts	(726)	(3,138)
Deferred charges and other assets, net and other long-term liabilities, net balances
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	1,929	2,023
Derivative Liabilities, commodity contracts		(463)
Significant Other Observable Inputs Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	40,432	31,383
Derivative Liabilities, commodity contracts	(726)	(3,601)
Significant Other Observable Inputs Level 2 | Fair liability and fair asset value of derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	38,503	29,360
Derivative Liabilities, commodity contracts	(726)	(3,138)
Significant Other Observable Inputs Level 2 | Deferred charges and other assets, net and other long-term liabilities, net balances
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	$ 1,929	2,023
Derivative Liabilities, commodity contracts		$ (463)