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Fair Value Measurements - Quantative Information about Level 3 Fair Value (Details) - Option Model - Implied Volatility - Fuel derivatives
Jun. 30, 2017
Third quarter 2017  
Quantitative Information About Level 3  
Fair Value Measurement Range, Minimum 14.00%
Fair Value Measurement Range, Maximum 29.00%
Fourth quarter 2017  
Quantitative Information About Level 3  
Fair Value Measurement Range, Minimum 20.00%
Fair Value Measurement Range, Maximum 31.00%
2018  
Quantitative Information About Level 3  
Fair Value Measurement Range, Minimum 22.00%
Fair Value Measurement Range, Maximum 30.00%
2019  
Quantitative Information About Level 3  
Fair Value Measurement Range, Minimum 16.00%
Fair Value Measurement Range, Maximum 25.00%
2020  
Quantitative Information About Level 3  
Fair Value Measurement Range, Minimum 16.00%
Fair Value Measurement Range, Maximum 21.00%