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Effect of Derivative Instruments on Statement of Operations (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Amount of (Gain) or Loss Unrealized, commodity contracts $ 1,910 $ 11,225 $ 6,428 $ 1,362
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Cost of product
        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Amount of (Gain) or Loss Recognized, commodity contracts 2,531 [1] 359 [1] 15,951 [1] 15,683 [1]
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Cost of installations and service
        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Amount of (Gain) or Loss Recognized, commodity contracts (67) [1] (120) [1] (401) [1] (224) [1]
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Delivery and branch expenses
        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Amount of (Gain) or Loss Recognized, commodity contracts $ (43) [1] $ 4 [1] $ (246) [1] $ (87) [1]
[1] Represents realized closed positions and includes the cost of options as they expire.