Effect of Derivative Instruments on Statement of Operations (Detail) - USD ($) $ in Thousands		3 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (Gain) or Loss Unrealized, commodity contracts		$ (8,551)	$ 5,536
Open Position
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (Gain) or Loss Unrealized, commodity contracts		(8,551)	5,536
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Cost of product | Closed Positions
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (Gain) or Loss Recognized, commodity contracts	[1]	3,381	(3,434)
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Cost of installations and service | Closed Positions
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (Gain) or Loss Recognized, commodity contracts	[1]	(94)	226
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Delivery and branch expenses | Closed Positions
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (Gain) or Loss Recognized, commodity contracts	[1]	$ (117)	$ 315

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