Fair Value Measurements - Quantative Information about Level 3 Fair Value (Details) - Significant unobservable inputs (Level 3) - Fuel derivatives - Measurement Input, Option Volatility	Sep. 30, 2018
Minimum | Fourth quarter 2018
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.16
Minimum | 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	0.21
Minimum | 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.19
Minimum | 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.19
Maximum | Fourth quarter 2018
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.29
Maximum | 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.29
Maximum | 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.26
Maximum | 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.22
Maximum | 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset, Measurement Input	0.19