Summary of Significant Accounting Policies - Frequent Flier Program (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) unit_of_accounting	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)
Accounting Policies [Abstract]
Credit shell, term of expiration	60 days
Signing bonus		$ 10.7
Number of units of accounting | unit_of_accounting	3
Affinity Card Program, deferred fair value of the transportation component, recognition period	17 months
Liability for future FREE SPIRIT award redemptions and unrecognized revenue from the sale of mileage credits	$ 13.8		$ 15.3