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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 6 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Mar. 31, 2019
Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of (Gain) or Loss Unrealized, commodity contracts [1] $ (13,401) $ 11,609 $ 17,638 $ 209
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] 14,345 (9,561) 8,193 (9,377)
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] 403 11 650 (571)
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] $ 271 $ (45) $ 437 $ (1,274)
[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.