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Fair Value Measurements - Quantative Information about Level 3 Fair Value (Details) - Option Model - Implied Volatility - Fuel derivatives	Jun. 30, 2018
Third quarter 2018
Quantitative Information About Level 3
Fair Value Measurement Range, Minimum	15.00%
Fair Value Measurement Range, Maximum	30.00%
Fourth quarter 2018
Quantitative Information About Level 3
Fair Value Measurement Range, Minimum	22.00%
Fair Value Measurement Range, Maximum	31.00%
2019
Quantitative Information About Level 3
Fair Value Measurement Range, Minimum	22.00%
Fair Value Measurement Range, Maximum	28.00%
2020
Quantitative Information About Level 3
Fair Value Measurement Range, Minimum	21.00%
Fair Value Measurement Range, Maximum	26.00%
2021
Quantitative Information About Level 3
Fair Value Measurement Range, Minimum	20.00%
Fair Value Measurement Range, Maximum	22.00%
2022
Quantitative Information About Level 3
Fair Value Measurement Range, Minimum	19.00%
Fair Value Measurement Range, Maximum	20.00%

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Quantitative information about Level 3 fair value measurements

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