Fair Value Measurements - Narrative (Details) - Fuel hedge contracts - Income Approach, Option Pricing Model - Option Volatility	Jun. 30, 2018
Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.16
Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.32

Value of input used to measure derivative liability.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

Name:	us-gaap_DerivativeLiabilityMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant