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Effect of Derivative Instruments on Statement of Operations (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of (Gain) or Loss Unrealized, commodity contracts [1] $ 3,135 $ (11,283) $ 7,026 $ (20,071)
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] 1,092 8,177 4,073 16,978
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]   72 (526) 848
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] $ 27 $ 150 $ (422) $ 1,029
[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.