DETAIL OF OTHER FINANCIAL STATEMENT CAPTIONS (DETAILS) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Detail of Other Financial Statement Captions [Line Items]
Property and transportation taxes	$ 71.5	$ 67.0
Receivables [Abstract]
Airline traffic receivables	66.5	53.6
Mileage Plan receivables	40.2	37.9
Other receivables	30.5	29.5
Allowance for doubtful accounts	(0.8)	(0.9)
Receivables - net	136.4	120.1
Prepaid Expenses and Other Current Assets [Abstract]
Prepaid aircraft rent	29.5	42.5
Prepaid engine maintenance	30.5	28.7
Prepaid credit card and travel agent commissions	9.8	8.9
Prepaid fuel	4.0	12.6
Other	19.2	14.0
Prepaid Expense and Other Current Assets	93.0	106.7
Other Assets [Abstract]
Restricted deposits (primarily restricted investments)	83.0	83.6
Prepaid aircraft rent	12.8	21.6
Deferred costs and other	32.0	31.1
Other Assets	127.8	136.3
Other Accrued Liablities (current) [Abstract]
Mileage Plan current liabilities	271.4	278.0
Other	148.6	143.0
Other Accrued Liabilities (current)	513.3	501.2
Postretirement medical liability [Member]
Other Accrued Liablities (current) [Abstract]
Pension and Postretirement Liability	4.1	4.9
Nonqualified Defined Benefit [Member]
Other Accrued Liablities (current) [Abstract]
Pension and Postretirement Liability	2.2	2.3
Commodity [Member]
Detail of Other Financial Statement Captions [Line Items]
Interest rate swaps	10.3	0
Interest rate swaps agreements [Member]
Detail of Other Financial Statement Captions [Line Items]
Interest rate swaps	$ 5.2	$ 6.0