Revenue Recognition - Narrative (Details) $ in Thousands	3 Months Ended						6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2018 USD ($) revenue_arrangement	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2018 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Air traffic liability	$ 357,645		$ 263,711				$ 357,645		$ 263,711	$ 220,184
Credit shell term							60 days
Frequent Flyer Program, number of types of revenue arrangements | revenue_arrangement							2
Revenue from Contract Contract with Customer	$ 851,771	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 1,555,909	$ 1,290,142	$ 2,643,552	$ 2,320,021
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Air traffic liability											$ 12,400
Mileage Credit Programs [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract Contract with Customer							$ 19,900	$ 25,000