Derivative Instruments, Hedging Activities and Fair Value Measurements, Derivative Fair Value Amounts (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	$ 84.8		$ 108.2
Liability Derivatives	246.6		321.2
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	37.4		87.9
Liability Derivatives	146.5		290.7
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	14.7		43.7
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	22.7		44.2
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	146.5		163.6
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0		127.1
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	47.4	[1]	20.3	[1]
Liability Derivatives	100.1	[1]	30.5	[1]
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	47.0		20.3
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0.4		0
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	100.1		30.3
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0		0.2
Derivatives not designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	42.5		47.0
Liability Derivatives	39.9		55.2
Derivatives not designated as hedging instruments [Member] | Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0		0
Liability Derivatives	20.6		20.7
Derivatives not designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0		0
Derivatives not designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0		0
Derivatives not designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	10.9		10.1
Derivatives not designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	9.7		10.6
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	42.5		47.0
Liability Derivatives	19.3		34.5
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	37.2		34.4
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	5.3		12.6
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	16.9		32.5
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	$ 2.4		$ 2.0

[1] Represents commodity derivative instrument transactions that have either not settled or have settled and not been invoiced. Settled and invoiced transactions are reflected in either accounts receivable or accounts payable depending on the outcome of the transaction.