Fair Value Measurements - Quantitative Information about Level 3 Fair Value (Details) - Significant unobservable inputs (Level 3) - Fuel derivatives - Measurement Input, Option Volatility	Sep. 30, 2020
Minimum | Fourth quarter 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.27
Minimum | 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.29
Minimum | 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.26
Minimum | Beyond 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.23
Weighted Average | Fourth quarter 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.41	[1]
Weighted Average | 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.33	[1]
Weighted Average | 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.27	[1]
Weighted Average | Beyond 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.25	[1]
Maximum | Fourth quarter 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.47
Maximum | 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.44
Maximum | 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.30
Maximum | Beyond 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.26

[1] Implied volatility weighted by the notional amount (barrels of fuel) that will settle in respective period.