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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, 2017 | Sep. 30, 2016
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Assets, commodity contracts | $ 8,725 | $ 13,061
Derivative Liabilities, commodity contracts | (2,817) | (10,555)
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 | 7,729 | 11,692
Other long-term liabilities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Assets, commodity contracts |  | 1,369
Derivative Liabilities, commodity contracts |  | (565)
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) | (9,990)
Cash Collateral |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Liabilities, commodity contracts | 0 | 0
Other long-term assets, net |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Assets, commodity contracts | 996 |
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) |
Quoted Prices in Active Markets for Identical Assets Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Assets, commodity contracts |  | 481
Derivative Liabilities, commodity contracts |  | (2,087)
Quoted Prices in Active Markets for Identical Assets Level 1 | Other long-term liabilities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Assets, commodity contracts |  | 481
Derivative Liabilities, commodity contracts |  | (484)
Quoted Prices in Active Markets for Identical Assets Level 1 | 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 |  | (1,603)
Quoted Prices in Active Markets for Identical Assets Level 1 | Cash Collateral |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Liabilities, commodity contracts | 0 | 0
Significant Other Observable Inputs Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Assets, commodity contracts | 8,725 | 12,580
Derivative Liabilities, commodity contracts | (2,817) | (8,468)
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 | 7,729 | 11,692
Significant Other Observable Inputs Level 2 | Other long-term liabilities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Assets, commodity contracts |  | 888
Derivative Liabilities, commodity contracts |  | (81)
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) | (8,387)
Significant Other Observable Inputs Level 2 | Cash Collateral |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Liabilities, commodity contracts | 0 | $ 0
Significant Other Observable Inputs Level 2 | Other long-term assets, net |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Assets, commodity contracts | 996 |
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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