Revenue Recognition - Narrative (Details) $ in Thousands	3 Months Ended					12 Months Ended
	Mar. 31, 2018 USD ($) revenue_arrangement	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 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	$ 357,193	$ 263,711				$ 263,711	$ 220,200
Credit shell term	60 days
Frequent Flyer Program, number of types of revenue arrangements | revenue_arrangement	2
Cash sales from marketing agreements	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 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]
Cash sales from marketing agreements	$ 10,000				$ 11,700