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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 | Sep. 30, 2018 | Sep. 30, 2017
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Assets, commodity contracts | $ 18,616 | $ 8,725
Derivative Liabilities, commodity contracts | (103) | (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 | 17,710 | 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 | 906 | 996
Derivative Liabilities, commodity contracts | (103) |
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 | 18,616 | 8,725
Derivative Liabilities, commodity contracts | (103) | (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 | 17,710 | 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 | 906 | 996
Derivative Liabilities, commodity contracts | $ (103) |
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)

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