Fair Value Measurements - Quantitative Information about Level 3 Fair Value (Details) - Option Model - Implied Volatility - Fuel derivatives	Dec. 31, 2015
2016
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	26.00%
Fair Value Measurement Range, Maximum	47.00%
2017
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	25.00%
Fair Value Measurement Range, Maximum	37.00%
2018
Quantitative Information About Level 3 [Line Items]
Fair Value Measurement Range, Minimum	16.00%
Fair Value Measurement Range, Maximum	31.00%