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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 | Mar. 31, 2020 | Sep. 30, 2019
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Assets, commodity contracts | $ 37,170 | $ 15,290
Derivative Liabilities, commodity contracts | (51,381) | (23,805)
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 | 35,497 | 13,824
Derivative Liabilities, commodity contracts | (49,514) | (22,086)
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 | 1,673 | 1,466
Derivative Liabilities, commodity contracts | (1,867) | (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 | 37,170 | 15,290
Derivative Liabilities, commodity contracts | (51,381) | (23,805)
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 | 35,497 | 13,824
Derivative Liabilities, commodity contracts | (49,514) | (22,086)
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 | 1,673 | 1,466
Derivative Liabilities, commodity contracts | $ (1,867) | $ (1,719)

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