Commitments and Contingencies - Commitments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013 aircraft_engine
Commitments and Contingencies Disclosure [Abstract]
Number of spare aircraft engines ordered (in aircraft engine)	2
Aircraft and Related Flight Equipment [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Committed expenditures, remainder of 2013	248.0
Committed expenditures, 2014	354.0
Committed expenditures, 2015	524.0
Committed expenditures, 2016	501.0
Committed expenditures, 2017	607.0
Committed expenditures, 2018 and beyond	2,336
Reservation System Provider [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Committed expenditures, remainder of 2013	3.1
Committed expenditures, 2014	3.7
Committed expenditures, 2015	3.7
Committed expenditures, 2016	3.7
Committed expenditures, 2017	3.7
Committed expenditures, 2018	2.5
Airbus A320 [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircraft ordered (in aircraft)	102
Airbus A320, current model [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircraft ordered (in aircraft)	52
Airbus A320 NEO [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of aircraft ordered (in aircraft)	50