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Effect of Derivative Instruments on Statement of Operations (Detail) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Amount of (Gain) or Loss Unrealized, commodity contracts $ 4,187 $ 6,566 $ 6,775
Open Position      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Amount of (Gain) or Loss Unrealized, commodity contracts 4,187 6,566 6,775
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Cost of product | Closed Positions      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Amount of (Gain) or Loss Recognized, commodity contracts [1] 13,368 11,781 17,769
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Cost of installations and service | Closed Positions      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Amount of (Gain) or Loss Recognized, commodity contracts [1] 1,831 (202) (440)
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Delivery and branch expenses | Closed Positions      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Amount of (Gain) or Loss Recognized, commodity contracts [1] $ 2,098 $ (104) $ (286)
[1] Represents realized closed positions and includes the cost of options as they expire.