GENERAL AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES GENERAL AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Passenger, Mainline	$ 862.7	$ 774.2	$ 1,586.1	$ 1,433.5
Passenger, Regional	187.6	179.0	360.4	340.6
Total passenger revenue	1,050.3	953.2	1,946.5	1,774.1
Freight and mail	30.6	29.1	55.0	54.0
Other - net	132.3	127.9	251.0	247.3
Total Operating Revenues	1,213.2	1,110.2	2,252.5	2,075.4
Scenario, Previously Reported [Member]
Passenger, Mainline		819.9		1,522.3
Passenger, Regional		194.3		370.8
Total passenger revenue		1,014.2		1,893.1
Freight and mail		29.1		54.0
Other - net		66.9		128.3
Total Operating Revenues		$ 1,110.2		$ 2,075.4