GENERAL AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES GENERAL AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Passenger, Mainline	$ 904.6	$ 835.6	$ 2,490.7	$ 2,269.1
Passenger, Regional	198.1	196.1	558.5	536.7
Total passenger revenue	1,102.7	1,031.7	3,049.2	2,805.8
Freight and mail	29.6	29.9	84.6	83.9
Other - net	139.9	136.5	390.9	383.8
Total Operating Revenues	1,272.2	1,198.1	3,524.7	3,273.5
Scenario, Previously Reported [Member]
Passenger, Mainline		887.4		2,409.7
Passenger, Regional		213.4		584.2
Total passenger revenue		1,100.8		2,993.9
Freight and mail		29.9		83.9
Other - net		67.4		195.7
Total Operating Revenues		$ 1,198.1		$ 3,273.5