Fair Value Measurements - Quantitative Information about Level 3 Fair Value (Details) - Significant unobservable inputs (Level 3) - Fuel derivatives - Measurement Input, Option Volatility	Jun. 30, 2021
Minimum | Third quarter 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.17
Minimum | Fourth quarter 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.27
Minimum | 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.24
Minimum | 2023
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.23
Minimum | Beyond 2023
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.23
Weighted Average | Third quarter 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.26
Weighted Average | Fourth quarter 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.29
Weighted Average | 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.30
Weighted Average | 2023
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.25
Weighted Average | Beyond 2023
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.24
Maximum | Third quarter 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.33
Maximum | Fourth quarter 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.33
Maximum | 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.37
Maximum | 2023
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.28
Maximum | Beyond 2023
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input (in years)	0.25