Derivatives and Hedging-Disclosures and Fair Value Measurements - Company's 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	Dec. 31, 2019	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	$ 8,001	$ 15,290
Derivative Liabilities, commodity contracts	(8,777)	(23,805)
Fair asset and fair liability value of derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	7,106
Derivative Liabilities, commodity contracts	(7,752)
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	895
Derivative Liabilities, commodity contracts	(1,025)
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		13,824
Derivative Liabilities, commodity contracts		(22,086)
Deferred charges and other assets, net balance
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts		1,466
Derivative Liabilities, commodity contracts		(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	8,001	15,290
Derivative Liabilities, commodity contracts	(8,777)	(23,805)
Significant Other Observable Inputs Level 2 | Fair asset and fair liability value of derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	7,106
Derivative Liabilities, commodity contracts	(7,752)
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	895
Derivative Liabilities, commodity contracts	$ (1,025)
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		13,824
Derivative Liabilities, commodity contracts		(22,086)
Significant Other Observable Inputs Level 2 | Deferred charges and other assets, net balance
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts		1,466
Derivative Liabilities, commodity contracts		$ (1,719)