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Fair Value Measurements - Quantitative Information about Level 3 Fair Value (Details) - Option Model - Implied Volatility - Fuel derivatives	Dec. 31, 2017
2018
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	11.00%
Fair Value Measurement Range, Maximum	26.00%
2019
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	17.00%
Fair Value Measurement Range, Maximum	22.00%
2020
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	17.00%
Fair Value Measurement Range, Maximum	21.00%
Beyond 2020
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	18.00%
Fair Value Measurement Range, Maximum	20.00%

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- Definition

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- Definition

Minimum range used in level 3 fair value calculation

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Quantitative information about Level 3 fair value measurements

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