Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) $ in Thousands, gal in Millions	12 Months Ended
	Sep. 30, 2016 USD ($) gal	Sep. 30, 2015 USD ($) gal
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	7.9	12.3
Notional Value	$ 10,800	$ 18,300
Fair Value	(40)	(500)
Aggregated cash posted as collateral in normal course of business	2,900
Hedging positions and payable amounts secured under credit facility	$ 300	$ 15,300
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	6.8	7.2
Notional Value	$ 13,800	$ 17,100
Fair Value	$ 300	$ 100
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	6.2	5.5
Notional Value	$ 6,500	$ 6,500
Fair Value	$ 20	$ 200
Synthetic calls | Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	84.9	94.6
Notional Value	$ 133,500	$ 183,900
Fair Value	$ 1,300	$ (12,000)
Inventory
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	9.9	6.8
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Value	$ 14,300	$ 12,700
Fair Value	$ 1,200	$ (1,900)
Swap Contracts Sold
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	1.2	1.8
Notional Value	$ 1,700	$ 3,400
Fair Value	$ 200	$ (500)
Corresponding Long And Short Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	4.4	8.9
Notional Value	$ 6,700	$ 17,200
Fair Value	$ 500	$ (2,000)
Long Swap Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	21.8	32.0
Notional Value	$ 32,200	$ 54,100
Fair Value	$ (2,100)	$ 2,900
Hedge its Internal Fuel Usage and Other Related Activities Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	5.7	6.0
Notional Value	$ 7,700	$ 11,100
Fair Value	$ 1,100	$ (1,900)