Fair Value Measurements - Quantitative Information about Level 3 Fair Value (Details) - Significant unobservable inputs (Level 3) - Fuel derivatives - Measurement Input, Option Volatility	Jun. 30, 2019
Minimum | Third quarter 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.24
Minimum | Fourth quarter 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.22
Minimum | 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.20
Minimum | 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.18
Minimum | 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.18
Maximum | Third quarter 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.3
Maximum | Fourth quarter 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.3
Maximum | 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.32
Maximum | 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.23
Maximum | 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.21