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	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	$ 26,164	$ 15,290
Derivative Liabilities, commodity contracts	(37,490)	(23,805)
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	24,274	13,824
Derivative Liabilities, commodity contracts	(35,711)	(22,086)
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	1,890	1,466
Derivative Liabilities, commodity contracts	(1,779)	(1,719)
Significant Other Observable Inputs Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	26,164	15,290
Derivative Liabilities, commodity contracts	(37,490)	(23,805)
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	24,274	13,824
Derivative Liabilities, commodity contracts	(35,711)	(22,086)
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	1,890	1,466
Derivative Liabilities, commodity contracts	$ (1,779)	$ (1,719)