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Effect of Derivative Instruments on Statement of Operations (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of (Gain) or Loss Unrealized, commodity contracts | [1] | $ (11,408) | $ (2,193) | $ (18,217)
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Cost of product |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of (Gain) or Loss Recognized, commodity contracts | [2] | 10,379 | 6,386 | 16,977
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Cost of installations and service |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of (Gain) or Loss Recognized, commodity contracts | [2] | (726) | (526) | 949
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Delivery and branch expenses |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of (Gain) or Loss Recognized, commodity contracts | [2] | $ (1,403) | $ (422) | $ 2,405
[1] | Represents the change in value of unrealized open positions and expired options.
[2] | Represents realized closed positions and includes the cost of options as they expire.

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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959 + Details
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- Definition The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 + Details
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