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Derivatives and Hedging - Disclosures and Fair Value Measurements - Effect of Derivative Instruments on Statement of Operations (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of (Gain) or Loss Unrealized, commodity contracts [1] $ 1,630 $ (7,515) $ 19,268 $ (7,306)
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] 558 914 8,751 (8,463)
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] 79 (102) 729 (673)
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] $ 75 $ (61) $ 512 $ (1,335)
[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.