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Effect of Derivative Instruments on Statement of Operations (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amount of (Gain) or Loss Unrealized, commodity contracts $ (5,458) $ 7,965
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 5,311 [1] 4,876 [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 (8) [1] (89) [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 $ (39) [1] $ (85) [1]
[1] Represents realized closed positions and includes the cost of options as they expire.