Fair Value Measurements - Quantitative Information about Level 3 Fair Value (Details) - Fuel derivatives - Significant unobservable inputs (Level 3) - Measurement Input, Option Volatility [Member]	Dec. 31, 2020
2021 | Maximum
Quantitative Information About Level 3 [Line Items]
Derivative Asset, Measurement Input	0.43
2021 | Weighted Average [Member]
Quantitative Information About Level 3 [Line Items]
Derivative Asset, Measurement Input	0.33
2021 | Minimum
Quantitative Information About Level 3 [Line Items]
Derivative Asset, Measurement Input	0.29
2022 | Maximum
Quantitative Information About Level 3 [Line Items]
Derivative Asset, Measurement Input	0.32
2022 | Weighted Average [Member]
Quantitative Information About Level 3 [Line Items]
Derivative Asset, Measurement Input	0.28
2022 | Minimum
Quantitative Information About Level 3 [Line Items]
Derivative Asset, Measurement Input	0.25
2023 | Maximum
Quantitative Information About Level 3 [Line Items]
Derivative Asset, Measurement Input	0.26
2023 | Weighted Average [Member]
Quantitative Information About Level 3 [Line Items]
Derivative Asset, Measurement Input	0.25
2023 | Minimum
Quantitative Information About Level 3 [Line Items]
Derivative Asset, Measurement Input	0.23
Beyond 2023 | Maximum
Quantitative Information About Level 3 [Line Items]
Derivative Asset, Measurement Input	0.26
Beyond 2023 | Weighted Average [Member]
Quantitative Information About Level 3 [Line Items]
Derivative Asset, Measurement Input	0.25
Beyond 2023 | Minimum
Quantitative Information About Level 3 [Line Items]
Derivative Asset, Measurement Input	0.25