MILEAGE PLAN MILEAGE PLAN (DETAILS) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Mileage Plan [Line Items]
Accrued Liabilities, Current	$ 551.5		$ 513.3
Deferred Revenue	406.2		410.2
Other Liabilities, Noncurrent	184.1		176.7
Passenger Revenue	950.4	878.9
Other Revenue, Net	64.5	61.4
Total Operating Revenues	1,039.3	965.2
Liabilities From Mileage Plan [Member]
Mileage Plan [Line Items]
Accrued Liabilities, Current	283.4		271.4
Deferred Revenue	389.1		392.2
Other Liabilities, Noncurrent	16.7		16.9
Liabilities	689.2		680.5
Revenue From Mileage Plan [Member]
Mileage Plan [Line Items]
Passenger Revenue	42.6	50.3
Other Revenue, Net	47.5	45.6
Total Operating Revenues	$ 90.1	$ 95.9