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
	Jun. 30, 2012	Mar. 31, 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	$ 0.1	$ 0.3
Total gains (losses) realized or unrealized included in earnings, net	(3.6)	2.2
Settlements, net	(2.5)	(2.4)
Balance at end of period	$ (1.0)	$ 0.1