Effect of Derivative Instruments on Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Mar. 31, 2015		Mar. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount of (Gain) or Loss Unrealized, commodity contracts	$ (12,631)		$ 4,105		$ (4,341)		$ (1,353)
Open Position
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount of (Gain) or Loss Unrealized, commodity contracts	(12,631)		4,105		(4,341)		(1,353)
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	17,322	[1]	3,216	[1]	10,517	[1]	8,527	[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	859	[1]	(87)	[1]	1,345	[1]	(95)	[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	$ 991	[1]	$ (75)	[1]	$ 1,465	[1]	$ (114)	[1]

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