Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions	12 Months Ended
	Sep. 30, 2018 USD ($) gal	Sep. 30, 2017 USD ($) gal	Aug. 31, 2018 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Hedging positions and payable amounts secured under credit facility	$ 0	$ 100,000
Interest rate swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Value			$ 50,000,000
Fair Value	39,000
Percentage of market risk exposure of long term debt			50.00%
Prepaid expense and other current assets
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business	$ 1,200,000
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	3.2	6.1
Notional Value	$ 8,200,000	$ 13,600,000
Fair Value	$ 200,000	$ 100,000
Call Option | Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	85.4	79.0
Notional Value	$ 182,900,000	$ 134,300,000
Fair Value	$ 14,000,000	$ 4,100,000
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	5.6	8.1
Notional Value	$ 8,500,000	$ 8,800,000
Fair Value	$ 2,000	$ 2,000
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	11.7	15.6
Notional Value	$ 24,600,000	$ 25,500,000
Fair Value	$ 2,900,000	$ 2,300,000
Swap Contracts Sold
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	1.2	1.3
Notional Value	$ 2,600,000	$ 2,200,000
Fair Value	$ 200,000	$ 200,000
Long Swap Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal		16.8
Notional Value		$ 28,500,000
Fair Value		$ (1,900,000)
Purchased Futures Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	53.1	5.2
Notional Value	$ 114,300,000	$ 8,500,000
Fair Value	$ 9,700,000
Sold Futures Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	68.9	11.3
Notional Value	$ 148,800,000	$ 18,000,000
Fair Value	$ (12,600,000)
Hedge its Internal Fuel Usage and Other Related Activities Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	6.5	6.3
Notional Value	$ 13,700,000	$ 9,900,000
Fair Value	$ 1,000,000	$ 1,100,000
Hedge its Internal Fuel Usage and Other Related Activities Swap Contracts Bought | Call Option | Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	0.5
Notional Value	$ 1,000,000
Fair Value	$ 100,000