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Derivatives and Hedging-Disclosures and Fair Value Measurements - Effect of Derivative Instruments on Statement of Operations (Detail) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of (Gain) or Loss Unrealized, commodity contracts [1] $ 25,113 $ (11,408) $ (2,193)
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] 9,266 10,379 6,386
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] 836 (726) (526)
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] $ 596 $ (1,403) $ (422)
[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.