Fair Value Measurements - Narrative (Details) - Fuel hedge contracts	12 Months Ended
Dec. 31, 2017
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Expected volatility rate (percent)	28.00%
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Expected volatility rate (percent)	10.00%

Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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