DERIVATIVE INSTRUMENTS - FAIR VALUE OF HEDGE POSITIONS (DETAILS) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Interest Rate Swap [Member]
Derivative [Line Items]
Interest Rate Cash Flow Hedge Liability at Fair Value	$ 10.0
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	6.2
Call Option [Member] | Commodity Option [Member]
Derivative [Line Items]
Price Risk Derivatives, at Fair Value, Net	153.6	129.3
Commodity Option [Member]
Derivative [Line Items]
Derivative Premiums Capitalized	122.5	108.6
Swap [Member] | Commodity Option [Member]
Derivative [Line Items]
Price Risk Derivatives, at Fair Value, Net	2.7	2.0
Commodity Option [Member]
Derivative [Line Items]
Price Risk Derivatives, at Fair Value, Net	$ 156.3	$ 131.3