Summary of Significant Accounting Policies - Frequent Flyer Program (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) revenueArrangementType	Dec. 31, 2020 USD ($)
Accounting Policies [Abstract]
Frequent flyer program, number of types of revenue arrangements | revenueArrangementType	2
Unrecognized revenue from future FREE SPIRIT award redemptions and the sale of mileage credits | $	$ 42.7	$ 31.6