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Effect of Derivative Instruments on Statement of Operations (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2017
Mar. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of (Gain) or Loss Unrealized, commodity contracts $ 12,442 $ (14,324) $ 3,891 $ (8,788)
Open Position        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of (Gain) or Loss Unrealized, commodity contracts [1] 12,442 (14,324) 3,891 (8,788)
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Cost of product | Closed Positions        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of (Gain) or Loss Recognized, commodity contracts [2] (400) 12,234 2,981 8,801
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Cost of installations and service | Closed Positions        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of (Gain) or Loss Recognized, commodity contracts [2] (432) 550 (526) 776
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Delivery and branch expenses | Closed Positions        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of (Gain) or Loss Recognized, commodity contracts [2] $ (332) $ 565 $ (449) $ 879
[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.