DERIVATIVE INSTRUMENTS - FAIR VALUE OF HEDGE POSITIONS (DETAILS) (USD $) In Millions, unless otherwise specified	Sep. 30, 2011	Dec. 31, 2010
Fair Value, Inputs, Level 2 [Member] | Call Option [Member] | Commodity Option [Member]
Derivative [Line Items]
Price Risk Derivatives, at Fair Value, Net	$ 75.2	$ 129.3
Fair Value, Inputs, Level 2 [Member] | Swap [Member] | Commodity Option [Member]
Derivative [Line Items]
Price Risk Derivatives, at Fair Value, Net	3.0	2.0
Fair Value, Inputs, Level 2 [Member] | Commodity Option [Member]
Derivative [Line Items]
Price Risk Derivatives, at Fair Value, Net	78.2	131.3
B737-800 [Member]
Derivative [Line Items]
Capital Leased Assets, Number of B737-800 Aircraft	6
Interest Rate Swap [Member]
Derivative [Line Items]
Interest Rate Cash Flow Hedge Liability at Fair Value	28.2
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	6.2
Commodity Option [Member]
Derivative [Line Items]
Derivative Premiums Capitalized	$ 128.7	$ 108.6