Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Details) (Aircraft fuel derivatives [Member], USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012
Aircraft fuel derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ (1.0)	$ 0.3
Total gains (losses) realized or unrealized included in earnings, net	1.5	0.1
Settlements, net	(0.6)	(0.5)
Balance at end of period	$ (0.1)	$ (0.1)