Derivative Instruments, Hedging Activities and Fair Value Measurements, Derivative Fair Value Amounts (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	$ 69.5	$ 61.6
Liability Derivatives	51.9	45.5
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	82.2	90.5
Liability Derivatives	47.4	211.3
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	31.4	45.2
Liability Derivatives	0	175.4
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	16.5	19.6
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	14.9	25.6
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0	175.4
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0	0
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	50.8	45.3
Liability Derivatives	47.4	35.9
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	46.6	45.3
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	4.2	0
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	45.8	35.4
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	1.6	0.5
Derivatives not designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	18.7	16.3
Liability Derivatives	14.8	26.8
Derivatives not designated as hedging instruments [Member] | Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0	0
Liability Derivatives	10.3	17.2
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	10.3	12.2
Derivatives not designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0	5.0
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	18.7	16.3
Liability Derivatives	4.5	9.6
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	16.1	15.7
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	2.6	0.6
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	2.5	8.9
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	$ 2.0	$ 0.7