Effect of Derivative Instruments on Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount of (Gain) or Loss Unrealized, commodity contracts	$ 7,965		$ 7,118
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	4,876	[1]	(592)	[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	(89)	[1]	51	[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	$ (85)	[1]	$ 8	[1]

[1] Represents realized closed positions and includes the cost of options as they expire.