Derivative Instruments, Hedging Activities and Fair Value Measurements, Derivative Fair Value Amounts (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	$ 26.1	$ 32.6
Liability Derivatives	5.2	7.8
Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	39.8	41.3
Liability Derivatives	38.3	55.1
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	51.2	63.5
Liability Derivatives	35.4	46.8
Derivatives designated as hedging instruments [Member] \| Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	26.1	32.6
Liability Derivatives	0	0
Derivatives designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	16.7	20.2
Derivatives designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	9.4	12.4
Derivatives designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0	0
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	25.1	30.9
Liability Derivatives	35.4	46.8
Derivatives designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	25.1	30.9
Derivatives designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0	0
Derivatives designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	35.4	46.5
Derivatives designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0	0.3
Derivatives not designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	14.7	10.4
Liability Derivatives	8.1	16.1
Derivatives not designated as hedging instruments [Member] \| Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0	0
Liability Derivatives	5.2	7.8
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	5.2	7.8
Derivatives not designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0	0
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	14.7	10.4
Liability Derivatives	2.9	8.3
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	13.4	7.6
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	1.3	2.8
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	2.4	5.5
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	$ 0.5	$ 2.8