Summary of Significant Accounting Policies - Frequent Flier Program (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) revenue_arrangement	Dec. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Frequent flyer program, number of types of revenue arrangements | revenue_arrangement	2
Air traffic liability	$ 315,408	$ 291,981
Unrecognized revenue from future FREE SPIRIT award redemptions and the sale of mileage credits	$ 29,800	$ 27,400
Difference between Revenue Guidance in Effect before and after Topic 606 | ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Air traffic liability			$ 12,400