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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 | Dec. 31, 2018 | Sep. 30, 2018
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Assets, commodity contracts | $ 40,440 | $ 18,616
Derivative Liabilities, commodity contracts | (58,757) | (103)
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 | 38,247 | 17,710
Deferred charges and other assets, net balance |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Assets, commodity contracts | 2,193 | 906
Derivative Liabilities, commodity contracts | (2,445) | (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 | (56,312) |
Significant Other Observable Inputs Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Assets, commodity contracts | 40,440 | 18,616
Derivative Liabilities, commodity contracts | (58,757) | (103)
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 | 38,247 | 17,710
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 | 2,193 | 906
Derivative Liabilities, commodity contracts | (2,445) | $ (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 | $ (56,312) |

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