Summary of Significant Accounting Policies - Primary Components of Non-Ticket Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Product Information [Line Items]
Non-ticket revenue	$ 1,281,632	$ 1,121,335	$ 972,125
Baggage
Product Information [Line Items]
Non-ticket revenue	488,434	434,269	381,386
Passenger usage fee
Product Information [Line Items]
Non-ticket revenue	411,742	358,920	298,092
Advance seat selection
Product Information [Line Items]
Non-ticket revenue	131,821	110,966	97,786
Other
Product Information [Line Items]
Non-ticket revenue	$ 249,635	$ 217,180	$ 194,861