Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) $ in Thousands, gal in Millions	12 Months Ended
	Sep. 30, 2017 USD ($) gal	Sep. 30, 2016 USD ($) gal
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	11.3	7.9
Notional Value	$ 18,000	$ 10,800
Fair Value		(40)
Aggregated cash posted as collateral in normal course of business	500
Hedging positions and payable amounts secured under credit facility	$ 100	$ 300
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	6.1	6.8
Notional Value	$ 13,600	$ 13,800
Fair Value	$ 100	$ 300
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	8.1	6.2
Notional Value	$ 8,800	$ 6,500
Fair Value	$ 2	$ 20
Synthetic calls | Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	79.0	84.9
Notional Value	$ 134,300	$ 133,500
Fair Value	$ 4,100	$ 1,300
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	15.6	9.9
Notional Value	$ 25,500	$ 14,300
Fair Value	$ 2,300	$ 1,200
Swap Contracts Sold
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	1.3	1.2
Notional Value	$ 2,200	$ 1,700
Fair Value	$ 200	$ 200
Purchased Futures Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	5.2	4.4
Notional Value	$ 8,500	$ 6,700
Fair Value		$ 500
Long Swap Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	16.8	21.8
Notional Value	$ 28,500	$ 32,200
Fair Value	$ (1,900)	$ (2,100)
Hedge its Internal Fuel Usage and Other Related Activities Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	6.3	5.7
Notional Value	$ 9,900	$ 7,700
Fair Value	$ 1,100	$ 1,100