Fair Value Measurements - Quantitative Information about Level 3 Fair Value (Details) - Significant unobservable inputs (Level 3) - Fuel derivatives - Measurement Input, Option Volatility	Mar. 31, 2019
Minimum | Second quarter 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.14
Minimum | Third quarter 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.21
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	0.19
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	0.17
Maximum | Second quarter 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.29
Maximum | Third quarter 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.27
Maximum | Fourth quarter 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.27
Maximum | 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.27
Maximum | 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.21
Maximum | 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.18