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, 2017	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	$ 19,975	$ 8,725
Derivative Liabilities, commodity contracts	(6)	(2,817)
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	19,220	7,729
Deferred charges and other assets, net balance
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	755	996
Derivative Liabilities, commodity contracts	(6)
Fair liability and fair asset value of derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities, commodity contracts		(2,086)
Deferred charges and other assets, net and other long-term liabilities balances
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities, commodity contracts		(731)
Significant Other Observable Inputs Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	19,975	8,725
Derivative Liabilities, commodity contracts	(6)	(2,817)
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	19,220	7,729
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	755	996
Derivative Liabilities, commodity contracts	$ (6)
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 Liabilities, commodity contracts		(2,086)
Significant Other Observable Inputs Level 2 | Deferred charges and other assets, net and other long-term liabilities balances
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities, commodity contracts		$ (731)