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Fair Value Measurements - Quantative Information about Level 3 Fair Value (Details) - Option Model - Implied Volatility - Fuel derivatives | Sep. 30, 2017
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Fourth quarter 2017 |
Quantitative Information About Level 3 |
Fair Value Measurement Range, Minimum | 14.00%
Fair Value Measurement Range, Maximum | 30.00%
2018 |
Quantitative Information About Level 3 |
Fair Value Measurement Range, Minimum | 19.00%
Fair Value Measurement Range, Maximum | 28.00%
2019 |
Quantitative Information About Level 3 |
Fair Value Measurement Range, Minimum | 18.00%
Fair Value Measurement Range, Maximum | 23.00%
Beyond 2019 |
Quantitative Information About Level 3 |
Fair Value Measurement Range, Minimum | 17.00%
Fair Value Measurement Range, Maximum | 21.00%

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