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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, 2014
Mar. 31, 2013
Mar. 31, 2014
Mar. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Amount of (Gain) or Loss Unrealized, commodity contracts $ 4,105 $ (3,447) $ (1,353) $ 4,518
Open Position [Member]
        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Amount of (Gain) or Loss Unrealized, commodity contracts 4,105 (3,447) (1,353) 4,518
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 3,216 [1] 8,544 [1] 8,527 [1] 13,420 [1]
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 (87) [1] (245) [1] (95) [1] (334) [1]
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 $ (75) [1] $ (118) [1] $ (114) [1] $ (203) [1]
[1] Represents realized closed positions and includes the cost of options as they expire.