GENERAL AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - MILEAGE PLAN (DETAILS) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Mileage Plan [Line Items]
Other accrued liabilities	$ 513.3	$ 501.2
Other liabilities	176.7	123.0
Deferred revenue	410.2	403.5
Deferred revenue from Mileage Plan awards issued but not yet flown	38.5	43.0
Passenger revenues	3,950.7	3,488.6	3,107.4
Other-net revenues	258.4	237.5	196.5
Total Operating Revenues	4,317.8	3,832.3	3,399.8
Liabilities From Mileage Plan [Member]
Mileage Plan [Line Items]
Other accrued liabilities	271.4	278.0
Other liabilities	16.9	13.8
Deferred revenue	392.2	382.1
Liabilities	680.5	673.9
Revenue From Mileage Plan [Member]
Mileage Plan [Line Items]
Passenger revenues	200.7	189.5	182.1
Other-net revenues	194.9	183.3	151.5
Total Operating Revenues	395.6	372.8	333.6
Commission revenue	$ 138.2	$ 123.7	$ 96.8