Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Details) (Aircraft Fuel Derivatives, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Aircraft Fuel Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at end of period	$ 0.3	$ 0.3	$ 3.5
Balance at beginning of period	0.3	3.5	1.4
Total realized or unrealized gains (losses) included in earnings, net	(0.2)	4.2	3.5
Settlements, net	$ (0.2)	$ 7.4	$ 1.4