DERIVATIVE INSTRUMENTS - BALANCE SHEET CLASSIFICATION (DETAILS) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ 380.8	$ 390.0
Posted collateral	17.5	0.9
Fuel hedge contracts [Member] | Derivative Instruments Not Designated as Hedges [Member] | Fuel hedge contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets, Current	22.0	46.7
Derivative Liabilities, Current	0	(10.3)
Fuel hedge contracts [Member] | Derivative Instruments Not Designated as Hedges [Member] | Fuel hedge contracts, noncurrent assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets, Noncurrent	39.3	70.2
Interest rate swaps agreements [Member] | Derivative Instruments Designated as Hedges [Member] | Other accrued liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities, Current	(5.7)	(5.2)
Interest rate swaps agreements [Member] | Derivative Instruments Designated as Hedges [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities, Noncurrent	(26.3)	(23.6)
Interest rate swaps agreements [Member] | Derivative Instruments Designated as Hedges [Member] | Gains (losses) in AOCL [Member]
Derivatives, Fair Value [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(32.0)	(28.8)
Derivative Gain (Loss) in Accumulated Other Comprehensive Loss To Be Reclassified Within Next Twelve Months	$ 5.7