Derivative Instruments, Hedging Activities and Fair Value Measurements, Gains and Losses on Derivative Instruments and Related Hedged Items (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Derivatives in fair value hedging relationships [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain/(Loss) Recognized in Income on Derivative	$ (0.8)	$ (12.0)
Gain/(Loss) Recognized in Income on Hedged Item	1.5	10.0
Derivatives in fair value hedging relationships [Member] | Interest rate derivatives [Member] | Location - Interest expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain/(Loss) Recognized in Income on Derivative	(1.5)	(12.3)
Gain/(Loss) Recognized in Income on Hedged Item	1.1	11.3
Derivatives in fair value hedging relationships [Member] | Commodity derivatives [Member] | Location - Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain/(Loss) Recognized in Income on Derivative	0.7	0.3
Gain/(Loss) Recognized in Income on Hedged Item	0.4	(1.3)
Derivatives in cash flow hedging relationships [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Value Recognized in Other Comprehensive Income/(Loss) on Derivative (Effective Portion)	(30.7)	(137.3)
Gain/(Loss) Reclassified from Accumulated Other Comprehensive Income/(Loss) to Income (Effective Portion)	(24.7)	(70.4)
Gain/(Loss) Recognized in Income on Derivative (Ineffective Portion)	0.3	(0.1)
Derivatives in cash flow hedging relationships [Member] | Interest rate derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Value Recognized in Other Comprehensive Income/(Loss) on Derivative (Effective Portion)	28.9	14.1
Derivatives in cash flow hedging relationships [Member] | Interest rate derivatives [Member] | Location - Interest expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain/(Loss) Reclassified from Accumulated Other Comprehensive Income/(Loss) to Income (Effective Portion)	(2.7)	(1.5)
Accumulated Other Comprehensive Loss related to interest rate derivative instruments expected to be reclassified to earnings in interest expense over the next twelve months	(19.1)
Derivatives in cash flow hedging relationships [Member] | Commodity derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Accumulated Other Comprehensive Loss related to commodity derivative instruments expected to be reclassified to earnings over the next twelve months	(59.3)
Derivatives in cash flow hedging relationships [Member] | Commodity derivatives [Member] | Location - Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Value Recognized in Other Comprehensive Income/(Loss) on Derivative (Effective Portion)	(39.6)	(155.4)
Gain/(Loss) Reclassified from Accumulated Other Comprehensive Income/(Loss) to Income (Effective Portion)	(10.0)	(69.2)
Gain/(Loss) Recognized in Income on Derivative (Ineffective Portion)	0	(0.1)
Accumulated Other Comprehensive Loss related to commodity derivative instruments expected to be reclassified to earnings over the next twelve months	(41.1)
Derivatives in cash flow hedging relationships [Member] | Commodity derivatives [Member] | Location - Operating costs and expenses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Value Recognized in Other Comprehensive Income/(Loss) on Derivative (Effective Portion)	(20.0)	4.0
Gain/(Loss) Reclassified from Accumulated Other Comprehensive Income/(Loss) to Income (Effective Portion)	(12.0)	0.3
Gain/(Loss) Recognized in Income on Derivative (Ineffective Portion)	0.3	0
Accumulated Other Comprehensive Loss related to commodity derivative instruments expected to be reclassified to earnings over the next twelve months	(18.2)
Derivatives not designated as hedging instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain/(Loss) Recognized in Income on Derivative	15.8	1.7
Derivatives not designated as hedging instruments [Member] | Interest rate derivatives [Member] | Location - Interest expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain/(Loss) Recognized in Income on Derivative	(2.2)	(2.1)
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Location - Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain/(Loss) Recognized in Income on Derivative	20.8	3.8
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Location - Operating costs and expenses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain/(Loss) Recognized in Income on Derivative	$ (2.8)	$ 0