Summary of Significant Accounting Policies - Frequent Flier Program (Details) (USD $) In Millions, unless otherwise specified				12 Months Ended	3 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2012 Transportation Component	Mar. 31, 2011 Marketing Component	Dec. 31, 2010 Marketing Component
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Affinity Card Program, deferred fair value of the transportation component, recognition period				16 months
Affinity Card Program, administrator agreement, expiration settlement, amount						$ 5.0
Affinity Card Program, administrator agreement, expiration settlement, gain recognized					0.4	4.6
Affinity Card Program, co-branded Spirit credit card, contract term			5 years
Liability for future FREE SPIRIT award redemptions and unrecognized revenue from the sale of mileage credits	$ 2.4	$ 3.7