Summary of Significant Accounting Policies - Frequent Flier Program (Details) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) unit_of_accounting	Dec. 31, 2014 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		14 months
Liability for future FREE SPIRIT award redemptions and unrecognized revenue from the sale of mileage credits		$ 14.9	$ 2.8