Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions	3 Months Ended
	Dec. 31, 2018 USD ($) gal	Dec. 31, 2017 gal	Sep. 30, 2018 USD ($)	Aug. 31, 2018 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Hedging positions and payable amounts secured under credit facility | $	$ 20,200,000		$ 0
Prepaid expense and other current assets
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business | $	3,000,000
Interest rate swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Value | $				$ 50,000,000
Percentage of market risk exposure of long term debt				50.00%
Fair Value | $	$ (700,000)		$ 39,000
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	7.9	7.3
Call Option | Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	92.4	83.9
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	6.4	8.7
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	14.0	17.9
Future Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	18.3	27.8
Future Contracts | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	41.2	55.1
Hedge its Internal Fuel Usage and Other Related Activities Short Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	2.0
Hedge its Internal Fuel Usage and Other Related Activities Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	4.3	2.3