Effect of Derivative Instruments on Statement of Operations (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (Gain) or Loss Unrealized, commodity contracts	[1]	$ (7,515)	$ 3,135	$ (7,306)	$ 7,026
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Cost of product
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (Gain) or Loss Recognized, commodity contracts	[2]	914	1,092	(8,463)	4,073
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Cost of installations and service
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (Gain) or Loss Recognized, commodity contracts	[2]	(102)		(673)	(526)
Fair Value, Measurements, Recurring | Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 | Commodity Contract | Delivery and branch expenses
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (Gain) or Loss Recognized, commodity contracts	[2]	$ (61)	$ 27	$ (1,335)	$ (422)

[1] Represents the change in value of unrealized open positions and expired options.

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