INCOME TAXES - DEFERRED TAX ASSETS AND LIABILITIES (DETAILS) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Income Tax Disclosure [Abstract]
Excess of tax over book depreciation	$ 795.2		$ 660.0
Fuel hedge contracts	0		8.7
Other—net	16.9		16.9
Gross deferred tax liabilities	812.1		685.6
Mileage Plan	(241.9)		(237.2)
AMT and other tax credits	(53.1)		(57.5)
Inventory obsolescence	(13.4)		(16.5)
Deferred gains	(14.1)		(15.9)
Employee benefits	(217.6)		(179.1)
Loss carryforwards	(13.0)	[1]	(4.1)	[1]
Fuel hedge contracts	(2.8)		0
Other—net	(27.5)		(15.9)
Gross deferred tax assets	583.4		526.2
Net deferred tax liabilities	228.7		159.4
Current deferred tax asset	(134.2)		(120.5)
Noncurrent deferred tax liability	$ 362.9		$ 279.9

[1] The Federal loss carryforward of $27.7 million ($9.7 million tax effected) expires in 2031; State loss carryforwards of $59.0 million ($3.3 million tax effected) expire beginning in 2014 and ending in 2031.