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Effect of Derivative Instruments on Statement of Operations (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 6 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Mar. 31, 2013
Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Amount of (Gain) or Loss Unrealized, commodity contracts $ (3,447) $ (16,981) $ 4,518 $ (9,863)
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 8,544 [1] 15,916 [1] 13,420 [1] 15,324 [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 (245) [1] (156) [1] (334) [1] (104) [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 $ (118) [1] $ (98) [1] $ (203) [1] $ (90) [1]
[1] Represents realized closed positions and includes the cost of options as they expire.